The theory of endomorphism semigroups of groups is quite modestly developed. There are known only some general facts on this topics and a series of groups that are determined by their endomorphism semigroups in the class of all groups. In the first part of this book, some general facts on endomorphism semigroups of groups are presented with full proofs. In the second part of the book, four classes of finite groups of order 32 are described by properties of their endomorphism semigroups. It is proved that each group of these classes is determined by its endomorphism semigroup in the class of all groups.