Obsessions are persistent ideas, thoughts, impulses, or images that are seen as intrusive and not appropriate. They cause anxiety and distress. There is a sense that the obsession is somehow 'alien' to the person and not under his or her control. The most common obsessions are: 1) thoughts about contamination of some kind, 2) doubts about whether an important action has been performed (like locking the door on leaving the house), 3) the need to have things in a particular order, 4) aggressive or horrible images, or 5) sexual images. The person with OCD typically trying to ignore or suppress these thoughts. When that doesn't work, she or he turns to some competing behavior to neutralize the obsession. These are the compulsions. Compulsions are repetitive behaviors such as hand washing, ordering, checking) or mental acts (praying, counting, repeating words silently). The compulsions can consume enormous amounts of time, and usually interfere with other important parts of a person's life.

OCD was previously thought to be a rare disorder. In recent years, however, research has found that about 2.5% of the population will suffer from OCD during their lifetime. In any single year, about 1.5 - 2% of the population have this disorder.