On almost split sequences

This thesis is based on studies on almost split sequences, a special type of short exact sequences that play an important role in the representation theory of artin algebras. These sequences were first defined by M. Auslander and I. Reiten in 1974, and some of their properties were examined in a series of three articles by the same authors in 1977. In the introduction part of the thesis, which consists of three parts, information about the historical development of the thesis subject is given. The second chapter includes the definitions of the necessary concepts and some basic features for the thesis work. In the third part; the properties of the radical of the module category on a finite-dimensional algebra are expressed. Then, the concepts of almost split morphism and minimal morphism are given and their relations with irreducible mor- phisms are examined. In the third part, the definition and some characterizations of the almost split sequences are given. Finally, the existence of almost split sequences is proved by a functorial approach, close to the original spirit of the Auslander-Reiten theory.