"Obsession" is a second season episode of the American science fiction television series, Star Trek and was broadcast on December 15, 1967. It is episode No. 42, production No. 47, written by Art Wallace, and directed by Ralph Senensky.

In the episode, Captain James T. Kirk becomes obsessed with destroying a deadly entity he once encountered in his youth.

The episode deals with several human emotions: guilt, fear, obsession and, ultimately, redemption. While Captain Kirk is the primary focus, the viewer sees glimpses of these emotions in other members of the ship. At various times the "fight or flight" instinct is shown as an internal battle which one has to overcome in order to find emotional peace.

The Federation starship USS Enterprise has sent a landing team to a planet to search for tritanium deposits. While surveying, Captain Kirk notices a sweet honey smell and has his men go on alert. He tells them to scan for dikironium and immediately fire at anything that looks like a gaseous cloud. The landing party fans out to explore.

One group picks up a signal but loses it, at which point they are attacked. Kirk and Science Officer Spock rush over to the scene, finding three crewmen down; two dead, the other, Ensign Rizzo, near death. All of the men have had their red blood cells drained of hemoglobin. Kirk fears that the killer is the same entity he had encountered 11 years before while serving aboard the USS Farragut. As a young lieutenant, Kirk had hesitated in firing the ship's phasers at the creature, which attacked. The encounter killed half the Farragut crew (including Captain Garrovick, who was a hero and role model to Kirk), with Kirk and the ship's first officer among the survivors. Even though Kirk was given a commendation by the first officer for bravery in battle, Kirk has felt terrible guilt over this incident ever since.

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