This document summarizes defense mechanisms, which are classified into successful and unsuccessful categories. Successful mechanisms like repression, rationalization, and intellectualization help deal with reality. Unsuccessful mechanisms like suppression, reaction formation, and denial are compensatory when used in moderation but can cause problems when used excessively. Specific defense mechanisms are defined, such as repression being the unconscious forgetting of painful thoughts, and projection being the transferring of unacceptable thoughts onto others.