Robert Langford Modini Stack (13 January 1919 – 14 May 2003) was an Emmy Award-winning and Academy Award nominated American stage and movie actor. He was perhaps best known for his film acting as well as his role in the television series The Untouchables and as host of Unsolved Mysteries. He starred in more than 40 films.

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 reached 20-years-old, Stack achieved minor fame as a sportsman. He was an avid polo player. 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.