This chapter describes the nature of depression, explaining that this disorder is not only prevalent in the population, but is also characterized by relapse and recurrence. Cognitive theories of depression share the idea that individual differences in maladaptive thinking and negative appraisals of life stress account for the disorder. Most contemporary cognitive models of depression have involved refinements and expansions of Beck's original theory. Cognitive therapy aims to help individuals shift their cognitive appraisals from ones that are unhealthy and maladaptive to ones that are more evidence-based and adaptive. The effectiveness of cognitive-behavioral therapy (CBT) is based on the extent to which patients learn to use the skills conveyed in therapy outside of the actual session. CBT is well supported for the treatment of an acute episode of depression and serves as a prophylaxis against subsequent episodes.