In Search of... is an American television series that was broadcast weekly from 1976 to 1982, devoted to mysterious phenomena. It was created after the success of three one-hour documentaries produced by creator Alan Landsburg: In Search of Ancient Astronauts in 1973 (based on the 1968 book/ 1970 film Chariots of the Gods? by Erich von Däniken), In Search of Ancient Mysteries (1974), and The Outer Space Connection in 1975 (later adapted into popular paperbacks written by Landsburg), all of which featured narration by Rod Serling, who was the initial choice to host the spin-off show. Serling died before production started, and Leonard Nimoy was then selected to be the host. The series was revived with host Mitch Pileggi in 2002 and again in 2018 with Zachary Quinto for the History channel.

The original series was shown in Australia in the 1980s under the title Great Mysteries of the World, with each episode having an introduction and conclusion presented by television presenter Scott Lambert.

Each episode of the program explored a general or specific topic in one of several general categories as given in the opening titles: Extraterrestrials, Magic & Witchcraft, Missing Persons, Myths & Monsters, Lost Civilizations, and Special Phenomena (changed to Strange Phenomena from season 3 onward). The program conducted investigations into the controversial and paranormal (e.g., UFOs, Bigfoot, and the Loch Ness Monster). Additionally, it featured episodes about mysterious historical events and personalities such as Anna Anderson, the Lincoln Assassination, the Jack the Ripper murders, infamous cults (e.g., Jim Jones), and missing persons, cities, and ships (e.g., Amelia Earhart, Jimmy Hoffa, D. B. Cooper, the Mary Celeste, the Titanic, the lost Roanoke Colony). Each episode's opening credits included a verbal disclaimer about the conjectural nature of the evidence and theories to be presented:

This series presents information based in part on theory and conjecture. The producer's purpose is to suggest some possible explanations, but not necessarily the only ones, to the mysteries we will examine.

The program included interviews, reenactments, and host segments including Nimoy on location or in studio, and each episode featured voiceovers by Nimoy. The music was composed by Laurin M. Rinder and W. Michael Lewis. A soundtrack album was released on AVI (American Variety International) Records in 1977.

Nimoy wrote an episode about the turbulent life of artist Vincent van Gogh, having earlier played the artist's brother Theo in a one-man show. As part of his research, Nimoy found records in the archives of the hospital where Van Gogh was treated that suggested that he suffered from epilepsy rather than insanity.

The show also spawned at least six spin-off books, all written by Landsburg with forewords by Nimoy: In Search of Lost Civilizations, In Search of Extraterrestrials, In Search of Magic and Witchcraft, In Search of Strange Phenomena, In Search of Missing Persons, and In Search of Myths and Monsters, with an additional book that collected the best segments from these existing volumes.

In 1978, Landsburg produced a Bigfoot documentary using portions of two In Search of... episodes ("The Monster Hunters" and "The Yeti") called Manbeast! Myth or Monster, based on his book In Search of Myths and Monsters. Though Nimoy had written the foreword to Landsburg's book, he did not narrate this documentary.