Mariska Hargitay & Peter Hermann on the secret to their lasting marriage (Exclusive)

After 13 years of marriage, Mariska Hargitay and her husband, Peter Hermann, are opening up about the key to their long-lasting love -- and how sports analogies have helped!

“There is no secret,” Hermann, 50, told ET during a sit-down interview with Hargitay to promote his new children’s book, If the S in Moose Comes Loose. “We’re all just working through it -- whether it’s a relationship that exists in the public eye to some degree or doesn’t. Our son plays basketball and his coach says, ‘Fundamentals, fundamentals, fundamentals,’ and I think when it comes to relationships, the fundamentals, in the end, are not that complicated.”

“That doesn’t mean that they’re easy, but they’re not that complicated,” he added. “It's fundamentals -- kindness, listen well and fight fair."

See photos of Mariska Hargitay and Peter Hermann together:

63 PHOTOS
Mariska Hargitay and Peter Hermann together through the years
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Mariska Hargitay and Peter Hermann together through the years
NEW YORK - JULY 26: Actress Mariska Hargitay and boyfriend Peter Hermann attend Super Saturday 6, the annual designer sale and family event to benefit the Ovarian Cancer Research Fund at Nova's Ark Project July 26, 2003 in Water Mill, NY. (Photo by Evan Agostini/Getty Images)
Mariska Hargitay and Peter Hermann during Entertainment Weekly Hosts 10th Annual Viewing Party at Elaine's in New York City, New York, United States. (Photo by Stephen Lovekin/FilmMagic)
NEW YORK - MARCH 7: Actress Mariska Hargitay and date Peter Hermann attend the premiere of 'Secret Window' March 7, 2004 in New York City. (Photo by Peter Kramer/Getty Images)
Mariska Hargitay and Peter Hermann during The 3rd Annual 'Children's Day Artrageous' at The Metropolitan Pavilion in New York City, New York, United States. (Photo by Djamilla Rosa Cochran/WireImage)
Peter Hermann and Mariska Hargitay, winner of Best Female Lead Actor in a Drama Series (Photo by L. Busacca/WireImage for AWRT)
Mariska Hargitay and husband Peter Hermann during The 56th Annual Primetime Emmy Awards - Governors Ball at The Shrine Auditorium in Los Angeles, California, United States. (Photo by SGranitz/WireImage)
Mariska Hargitay and husband Peter Hermann during ''Law & Order'' Season Premiere Party - Arrivals at One Little West 12th Street Restaurant in New York City, New York, United States. (Photo by James Devaney/WireImage)
NEW YORK - DECEMBER 1: Actress Mariska Hargitay and Peter Hermann arrive at the 100 Women Against Child Abuse Fourth Annual Benefit on December 1, 2004 at Cipriani's in New York City. (Photo by Brad Barket/Getty Images)
NEW YORK - DECEMBER 13: Actress Mariska Hargitay and her husband, Peter Hermann, arrive for a gala to honor leaders in tourism sponsored by NYC & Company at the Museum of Modern Art December 13, 2004 in New York City. (Photo by Scott Gries/Getty Images)
Mariska Hargitay and husband Peter Hermann during Entertainment Weekly 11th Annual Oscar Viewing Party at Elaines Restaurant in New York City, New York, United States. (Photo by Robin Platzer/FilmMagic)
NEW YORK CITY, NY - APRIL 27: Mariska Hargitay and Peter Hermann attend TUPPERWARE Throws a Downtown 'Ice-T' Party with 'Law & Order' Stars to Benefit the Boys and Girls Clubs of Mt. Vernon at KOS Lounge on April 27, 2005 in New York City. (Photo by Jimi Celeste/Patrick McMullan via Getty Images)
NEW YORK - MAY 02: Actress Mariska Hargitay and husband Peter Hermann attend the MET Costume Institute Gala Celebrating Chanel at the Metropolitan Museum of Art May 2, 2005 In New York City. (Photo by Evan Agostini/Getty Images)
Mariska Hargitay and Peter Hermann during 2005 NBC Network All Star Celebration - Arrivals at Century Club in Los Angeles, California, United States. (Photo by Gregg DeGuire/WireImage)
Peter Hermann and Mariska Hargitay (Photo by Jamie McCarthy/WireImage for InStyle Magazine)
Peter Hermann, Mariska Hargitay during Outside arrivals for the NYC premiere of 'War of the Worlds' at The Ziegfeld Theater in New York, New York, United States. (Photo by Sylvain Gaboury/FilmMagic)
Peter Hermann and Mariska Hargitay during 2005 Fashion Rocks - Red Carpet at Radio City Music Hall in New York City, New York, United States. (Photo by Robin Platzer/FilmMagic)
Mariska Hargitay and Peter Hermann (Photo by Jeffrey Mayer/WireImage)
Peter Hermann and Mariska Hargitay during Entertainment Weekly Magazine 4th Annual Pre-Emmy Party - Inside at Republic in Los Angeles, California, United States. (Photo by J.Sciulli/WireImage)
Mariska Hargitay and Peter Hermann during 58th Annual Primetime Emmy Awards - Governors Ball at The Shrine Auditorium in Los Angeles, California, United States. (Photo by Mathew Imaging/FilmMagic)
Peter Hermann and Mariska Hargitay 12864_km0190.JPG (Photo by KMazur/WireImage)
NEW YORK CITY, NY - JULY 24: Mariska Hargitay and Peter Hermann attend CONVERSATIONS ON THE CIRCLE With Senator Barack Obama And Dick Parsons at Time Warner Headquarters on July 24, 2007 in New York City. (Photo by PATRICK MCMULLAN/Patrick McMullan via Getty Images)
Actress Mariska Hargitay and husband Peter Hermann arrive at the 59th Primetime Emmy Awards Nominees Reception at Wolfgang Puck at the Pacific Design Center on September 14, 2007 in West Hollywood, California. (Photo by Jeffrey Mayer/WireImage)
NEW YORK CITY, NY - JANUARY 28: Mariska Hargitay and Peter Hermann attend State of the Union Dinner at The Re-Opening of the Plaza Hotel Ballroom on January 28, 2008 in New York City. (Photo by Patrick McMullan/Patrick McMullan via Getty Images)
NEW YORK - FEBRUARY 06: Peter Hermann and Actress Mariska Hargitay attends the 'Betrayed' Opening Night - Arrivals at Culture Project on February 6, 2008 in New York City. (Photo by Jim Spellman/WireImage)
NEW YORK - MARCH 17: Actor Peter Hermann and actress Mariska Hargitay attends the 19th Annual GLAAD Media Awards at the Marriott Marquis on March 17, 2008 in New York City. (Photo by Dimitrios Kambouris/WireImage)
�2008 credit NCNA only Mariska Hargitay, a cast member in the motion picture comedy 'The Love Guru', arrives with her husband Peter Hermann for the premiere of the film at Grauman's Chinese Theatre in the Hollywood section of Los Angeles on June 11, 2008. XYZ (Photo by Philip Ramey/Corbis via Getty Images)
NEW YORK - MAY 20: Peter Hermann and actress Mariska Hargitay attend the Joyful Heart Foundation Gala at The IAC Building on May 20, 2008 in New York City. (Photo by Jason Kempin/WireImage)
NEW YORK - JULY 14: Actress Mariska Hargitay and Actors Peter Hermann attend the premiere of 'The Dark Knight' at AMC Loews Lincoln Center on July 14, 2008 in New York City. (Photo by Jim Spellman/WireImage)
Actress Mariska Hargitay and actor Peter Hermann arrive at the 60th Primetime Emmy Awards at the Nokia Theater on September 21, 2008 in Los Angeles, California. (Photo by Jeff Kravitz/FilmMagic)
NEW YORK - OCTOBER 15: Actress Mariska Hargity and husband Peter Hermann attend the 2008 Princess Grace Awards Gala at Cipriani 42nd Street on October 15, 2008 in New York City. (Photo by Matthew Peyton/Getty Images)
NEW YORK - OCTOBER 26: Peter Hermann and Actress Mariska Hargitay attend the Joyful Heart Foundation Dinner at Bon Appetit Supper Club and Cafe on October 26, 2008 in New York City. (Photo by Jemal Countess/WireImage for Bon Appetit Magazine)
Actress Mariska Hargitay and Peter Hermann arrive at the 61st Primetime Emmy Awards held at the Nokia Theatre on September 20, 2009 in Los Angeles, California. (Photo by Steve Granitz/WireImage)
NEW YORK CITY, NY - OCTOBER 20: Mariska Hargitay and Peter Hermann attend FOX SEARCHLIGHT PICTURES Proudly Presents the World Premiere of AMELIA at Paris Theatre on October 20, 2009 in New York City. (Photo by JIMI CELESTE/Patrick McMullan via Getty Images)
NEW YORK - DECEMBER 09: Actress Mariska Hargitay and Peter Hermann attend the New York premiere of 'It's Complicated' at The Paris Theatre on December 9, 2009 in New York City. (Photo by Stephen Lovekin/Getty Images)
Mariska Hargitay (R) and Peter Hermann arrive to the TNT/TBS broadcast of the 16th Annual Screen Actors Guild Awards held at the Shrine Auditorium on January 23, 2010 in Los Angeles, California. 19379_003_KM_0635.JPG (Photo by Kevin Mazur/WireImage)
NEW YORK, NY - APRIL 14: Mariska Hargitay and Peter Hermann attend the opening night after-party for 'War Horse' on Broadway at Avery Fisher Hall at Lincoln Center on April 14, 2011 in New York City. (Photo by Bruce Glikas/FilmMagic)
NEW YORK, NY - MAY 16: Peter Hermann (L) and Mariska Hargitay attend HBO's 'Too Big to Fail' premiere at The Museum of Modern Art on May 16, 2011 in New York City. (Photo by Rob Kim/FilmMagic)
EAST HAMPTON, NY - JULY 24: (L-R) Peter Hermann and Mariska Hargitay attend a cocktail party to celebrate the opening of Michael Stars hosted by Hamptons Magazine, The Joyful Heart Foundation, FIJI water and O.N.E Coconut water at the Michael Stars Store on July 24, 2010 in East Hampton, New York. (Photo by Astrid Stawiarz/WireImage for Niche Media)
NEW YORK, NY - DECEMBER 04: Mariska Hargitay and Peter Hermann attend the 'War Horse' world premiere at Avery Fisher Hall at Lincoln Center for the Performing Arts on December 4, 2011 in New York City. (Photo by Steve Mack/FilmMagic)
NEW YORK, NY - MAY 23: Actor Peter Hermann, actress Mariska Hargitay with their baby girl, Amaya Josephine attend the 20th Annual Playground Partners Family Party at Central Park, Heckscher Softball Fields on May 23, 2012 in New York City. (Photo by Slaven Vlasic/Getty Images)
NEW YORK, NY - APRIL 26: Peter Hermann and Mariska Hargitay attends the 'Leap Of Faith' Broadway Opening Night at St. James Theatre on April 26, 2012 in New York City. (Photo by John Lamparski/WireImage)
NEW YORK, NY - JUNE 06: August Hermann, Peter Hermann, Mariska Harigtay and daughter Amaya Hermann attends the 2012 Baby Buggy Bedtime Bash hosted by Jessica And Jerry Seinfeld on June 6, 2012 in New York City. (Photo by Gary Gershoff/WireImage)
NEW YORK, NY - SEPTEMBER 22: Mariska Hargitay (L) and Peter Hermann attend the 'Hotel Transylvania' premiere at the Academy Theater at Lighthouse International on September 22, 2012 in New York City. (Photo by Rob Kim/FilmMagic)
NEW YORK, NY - MAY 09: Peter Hermann and Mariska Hargitay attend the 2013 Joyful Heart Foundation gala at Cipriani 42nd Street on May 9, 2013 in New York City. (Photo by Michael N. Todaro/WireImage)
HOLLYWOOD, CA - SEPTEMBER 26: Actors Peter Hermann andMariska Hargitay attend The Joyful Heart Foundation Presents: JoyROCKS to celebrate the NO MORE PSA Launch at Milk Studios on September 26, 2013 in Hollywood, California. (Photo by Charley Gallay/Getty Images for Joyful Heart Foundation)
NEW YORK, NY - OCTOBER 06: Actress Mariska Hargitay and Peter Hermann attend 2nd Annual Mario Batali Foundation Honors Dinner at Del Posto Ristorante on October 6, 2013 in New York City. (Photo by Jim Spellman/WireImage)
HOLLYWOOD, CA - NOVEMBER 08: Actor Peter Hermann, son Andrew, daughter Amaya, actress Mariska Hargitay and son August attend Mariska Hargitay's Star ceremony on The Hollywood Walk of Fame held on November 8, 2013 in Hollywood, California. (Photo by Albert L. Ortega/Getty Images)
NEW YORK, NY - NOVEMBER 12: Actor Peter Hermann and actress Mariska Hargitay attend the premiere of 'Philomena' hosted by The Weinstein Company at the Paris Theater on November 12, 2013 in New York City. (Photo by Ben Gabbe/FilmMagic)
NEW YORK, NY - SEPTEMBER 4: Mariska Hargitay and her husband Peter Hermann attend the match between Roger Federer of Switzerland and Gael Monfils of France during Day 11 of the 2014 US Open at USTA Billie Jean King National Tennis Center on September 4, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Jean Catuffe/GC Images)
NEW YORK, NY - NOVEMBER 16: Mariska Hargitay and Peter Hermann attend the Broadway Opening Performance of 'The River' at Circle in the Square Theatre on November 16, 2014 in New York City. (Photo by Walter McBride/WireImage)
PHOENIX, AZ - JANUARY 31: Actors Peter Hermann (L) and Mariska Hargitay attend the 4th Annual NFL Honors at Phoenix Convention Center on January 31, 2015 in Phoenix, Arizona. (Photo by Kevin Mazur/WireImage)
NEW YORK, NY - AUGUST 06: Mariska Hargitay and Peter Hermann attend the 'Hamilton' Broadway Opening Night After Party at Pier 60 on August 6, 2015 in New York City. (Photo by Walter McBride/WireImage)
NEW YORK, NY - SEPTEMBER 13: Mariska Hargitay and her husband Peter Hermann arrive to attend the Men's Final on day fourteen of the 2015 US Open at USTA Billie Jean King National Tennis Center on September 13, 2015 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Jean Catuffe/GC Images)
NEW YORK, NY - JANUARY 12: Actors Mariska Hargitay (L) and Peter Hermann attend the 'Younger' Season 2 and 'Teachers' Series Premiere at The NoMad Hotel on January 12, 2016 in New York City. (Photo by Mike Coppola/Getty Images)
HONOLULU, HI - SEPTEMBER 4: Mariska Hargitay, left, and Peter Hermann arrive at the Joyful Mele An Evening To Benefit The Joyful Heart Foundation at the Kahala Hotel & Resort on September 04, 2016 in Honolulu, Hawaii. (Photo by Marco Garcia/Getty Images for The Joyful Heart Foundation)
EAST HAMPTON, NY - OCTOBER 09: Mariska Hargitay (L) and Peter Hermann attend the Chairman's Reception during the Hamptons International Film Festival 2016 at Stuart Match Suna's Residence on October 9, 2016 in East Hampton, New York. (Photo by Matthew Eisman/Getty Images for Hamptons International Film Festival)
NEW YORK, NY - APRIL 17: Peter Hermann and Mariska Hargitay attend the 'Groundhog Day' Broadway Opening Night at August Wilson Theatre on April 17, 2017 in New York City. (Photo by Dimitrios Kambouris/Getty Images)
NEW YORK, NY - JANUARY 11: Actors Mariska Hargita and Peter Hermann attend the TV Guide Celebrates Mariska Hargitay at Gansevoort Park Avenue on January 11, 2017 in New York City. (Photo by Nicholas Hunt/Getty Images)
NEW YORK, NY - MAY 22: Peter Hermann and Mariska Hargitay attend The Joyful Revolution Gala In New York City hosted by Mariska Hargitay's Joyful Heart Foundation on May 22, 2017 in New York City. (Photo by Dimitrios Kambouris/Getty Images for Joyful Heart Foundation)
NEW YORK, NY - SEPTEMBER 23: (L-R) Peter Hermann and Mariska Hargitay attend NBC & Vanity Fair host a party for 'Will & Grace' at Mr. Purple at the Hotel Indigo LES on September 23, 2017 in New York City. (Photo by Paul Bruinooge/Patrick McMullan via Getty Images)
NEW YORK, NEW YORK - JUNE 27: Peter Hermann and Mariska Hargitay attend the 'Younger' Season Four Premiere Party at Mr. Purple on June 27, 2017 in New York City. on June 27, 2017 in New York City. (Photo by Jamie McCarthy/Getty Images)
NEW YORK, NY - FEBRUARY 24: Mariska Hargitay and Peter Hermann sit courtside with children Amaya Hermann, Andrew Hermann and August Hermann at the New York Knicks vs Boston Celtics game at Madison Square Garden on February 24, 2018 in New York City. (Photo by James Devaney/Getty Images)
NEW YORK, NY - MARCH 17: Mariska Hargitay and Peter Hermann attend the book launch party for Peter Hermann's 'If The S in Moose Comes Loose' at Books of Wonder on March 17, 2018 in New York City. (Photo by Nicholas Hunt/Getty Images for HarperCollins Publishers)
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The pair tied the knot in 2004 and juggle parenting their three children with Hargitay’s long-running role as Detective Olivia Benson on NBC’s Law and Order: SVU and Hermann’s latest gig as the love interest of Sutton Foster's character, Liza Miller, on TV Land's Younger.

Hermann also just launched his book, which he started back when their eldest child, 11-year-old son August, was young.

“The first time that I got the galley [proofs], I spread them out all over my table and the first thing [the kids] did was climb all over them, so the pages have their blessing,” Hermann said. “It was funny because the first copy that I got, we sat there and looked at the envelope for a while and very slowly opened it and just had a quiet moment.”

As for how the children responded to seeing their names in ink in the book’s dedication, Hargitay still recalls the look on their faces and said it was a special moment for the family.

“We have these crazy tea parties that are just crazy. It's hard to explain, but the point is for the kids to see the book and also to see their names in the dedication,” Hargitay shared. “[It] was so deeply meaningful to them and I remember their faces … To be included in that was so cool and to know what a huge inspiration the kids were for Peter's fertile imagination was so fun because they know that so many things happened [and] little things they say were included. So that was really cool for them.”

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