Robert Langford Modini Stack (January 13, 1919 – May 14, 2003) was an American actor. In addition to acting in more than 40 films, he also appeared on the television series The Untouchables and later served as the host of Unsolved Mysteries.

Stack was born in Los Angeles, California, but spent his early childhood growing up in Europe. He became fluent in French and Italian at an early age, but he did not learn English until returning to Los Angeles. Raised by his mother, Mary Elizabeth (née Wood), Stack's parents divorced when Stack was a year old, and his father, James Langford Stack, a wealthy advertising agency owner, died when Stack was nine. Stack always spoke of his mother with the greatest respect and love. When he wrote his autobiography, Straight Shooting, he included a picture of him and his mother. He captioned it, "Me and my best girl." Stack's grandfather was an opera singer from Illinois named Charles Wood, who went by the name "Modini."

By the time he was twenty, Stack had achieved minor fame as a sportsman. He was an avid polo player and shooter. He and his brother won the International Outboard Motor Championships, in Venice, Italy; and, at age 16, he became a member of the All-American Skeet Team. He set two world records in skeet shooting and became National Champion. In 1971, he was inducted into the National Skeet Shooting Hall of Fame.

Stack was a regular columnist for Gun World magazine.

Stack took drama courses at the Bridgewater State College. His deep voice and good looks attracted producers in Hollywood. When Stack visited the lot of Universal Studios at age 20, producer Joe Pasternak offered him an opportunity to enter the business. Recalled Stack, "He said, 'How'd you like to be in pictures? We'll make a test with Helen Parrish, a little love scene.' Helen Parrish was a beautiful girl. 'Gee, that sounds keen,' I told him. I got the part." Stack's first film, which teamed him with Deanna Durbin, was First Love, in 1939; this film was considered controversial at the time. He was the first actor to give Durbin an on-screen kiss.

Stack won acclaim for his next role, The Mortal Storm (1940). He played a young man who joins the Nazi party. This film was among the first to speak out against Adolf Hitler. As a youth, Stack admitted that he had a crush on Carole Lombard and in 1942 he appeared with her in To Be or Not To Be. He admitted he was terrified going into this role. He credits Lombard with giving him many tips on acting and with being his mentor. Lombard was killed in a plane crash shortly before the film was released.

During World War II, Stack served as gunnery instructor in the United States Navy. He continued his film career and appeared in such films as Fighter Squadron (1948), A Date with Judy (1948) and Bwana Devil (1952). In 1954, Stack was given his most important movie role. He appeared opposite John Wayne in The High and the Mighty. Stack played the pilot of an airliner which comes apart under stress after the airliner encounters engine trouble.

In 1957, Stack was nominated for an Academy Award for Best Supporting Actor for Written on the Wind.

Stack depicted the crimefighting Eliot Ness in the television drama The Untouchables (1959–1963). The show portrayed the ongoing battle between gangsters and federal agents in a Prohibition-era Chicago. The show brought Stack a best actor Emmy Award in 1960.

Stack also starred in three other drama series, rotating the lead with Tony Franciosa and Gene Barry in the lavish The Name of the Game (1968–1971), Most Wanted, (1976) and Strike Force (1981). Interestingly, in The Name of the Game, he played a former federal agent turned true-crime journalist, evoking memories of his role as Ness. In both Most Wanted and Strike Force he played a tough, incorruptible police captain commanding an elite squad of special investigators, also evoking the Ness role. Eventually, he would reprise the role in a 1991 television movie, The Return of Eliot Ness.

Known for his steadfast, humorless demeanor, he made fun of his own persona in comedies such as 1941 (1979), Airplane! (1980), Caddyshack II (1988), Beavis and Butthead Do America (1996) and BASEketball (1998). He also provided the voice for the character Ultra Magnus in The Transformers: The Movie (1986).

Stack appeared in the television mini-series Hollywood Wives in 1985, and appeared in several episodes of the popular primetime soap opera Falcon Crest in 1986. Ironically, Stack's series Strike Force was scheduled opposite Falcon Crest, where it quickly folded.

He began hosting Unsolved Mysteries in 1987. He thought very highly of the interactive nature of the show, saying that it created a "symbiotic" relationship between viewer and program, and that the hotline was a great crime-solving tool. Unsolved Mysteries aired from 1987 to 2002, first as specials in 1987 (Stack did not host all the specials), then as a regular series on NBC (1988–1997), then on CBS (1997–1999) and finally on Lifetime (2001–2002). Stack served as the show's host during its entire original series run.