Many treatments of formal and informal reasoning in mathematical logic and artificial intelligence have been shaped in large part by u seldom acknowledged view: the view that the process of reasoning is the process of deriving new knowledge from old, the process of discovering new truths contained in known truths. The basic problem with the conventional view of reasoning stems from the monotonicity of the reasoning process. The truth maintenance systems solve this problem. Doyle' s Truth Maintenance System - TMS, de Kleer's assumption-based Truth Maintenance System - ATMS, ART's viewpoint mechanism, and BEST's context mechanism - MEKON will be described in this paper. TMS and ATMS represent two different approaches in the truth maintenance. and some variations of these basic ideas are implemented in many other truth maintenance systems. ART's viewpoint mechanism and MEKON are similar to the ATMS, but they can solve some problems that neither TMS nor ATMS can solve.