Leopold Samuel Marks, was an English writer, screenwriter, and cryptographer. During the Second World War he headed the codes office supporting resistance agents in occupied Europe for the secret Special Operations Executive organisation. After the war, Marks became a playwright and screenwriter, writing scripts that frequently utilised his war-time cryptographic experiences. He wrote the script for Peeping Tom, the controversial film directed by Michael Powell that had a disastrous effect on Powell's career, but was later described by Martin Scorsese as a masterpiece. In 1998, towards the end of his life, Marks published a personal history of his experiences during the war, Between Silk and Cyanide, which was critical of the leadership of SOE.

Peeping Tom is a 1960 British psychological horror-thriller film directed by Michael Powell, written by Leo Marks, and starring Carl Boehm, Anna Massey, and Moira Shearer. The film revolves around a serial killer who murders women while using a portable film camera to record their dying expressions of terror, putting his footage together into a snuff film used for his own self pleasure. Its title derives from the expression "peeping Tom", which describes a voyeur.