Cognitive therapy is based on the premise that cognition, the process of acquiring knowledge and forming beliefs, is a primary determinant of mood and behavior. After reviewing the basic theories of cognitive therapy, the authors discuss its application to the treatment of depression. They theorize that cognitive therapy can improve depressive mood and behavior by focusing on the depressed patient's cognitive disorder. This disorder includes impaired learning and memory function and a systematic negative bias in thinking. The authors note the encouraging results of preliminary outcome studies of this new treatment of depression; such results, they conclude, warrant further development and testing of cognitive therapy.