An obsession is an involuntary, irrational thought that occurs repeatedly. An obsession may be mild, as in those instances when we cannot stop wondering whether we remembered to turn off a faucet or lock a door before leaving home. In some cases, however, an obsession becomes so persistent that it markedly interferes with daily life. Severe obsessions often have a violent or sexual quality to them, such as the desire to burn the house or rape a neighbor, which makes the person feel guilty and horrified as much by the content of the thought as by its persistence.

A compulsion is an action that a person uncontrollably performs again and again, although she or he has no conscious desire to do so. The act is often senseless, such as looking under the bed several times before going to sleep or locking and unlocking the door several times before going out. There are two general categories of compulsions: checking rituals, such as looking under the bed, and contamination compulsions, such as hand washing. A person may show little anxiety when carrying out a compulsion. However, if she or he is prevented from carrying out the compulsive ritual, the person becomes extremely anxious.

Such ritual behavior is often both obsessive and compulsive. Constant hand washing, for example, may be caused by an obsessive preoccupation with germs. Although we all can remember times when a song lyric ran persistenlly through our minds, no matter what we did to get rid of it, or when we checked several times to make sure that the alarm clock was set, these minor obsessions and compulsions pass. A pathological obsession compulsion continues-day after day, year after year.