The aim of this monograph is to present some recent research work on the combinatorial aspects of the theory of semigroups which are of great inter est for both algebra and theoretical computer science. This research mainly concerns that part of combinatorics of finite and infinite words over a finite alphabet which is usually called the theory of "unavoidable" regularities. The unavoidable regularities ofsufficiently large words over a finite alpha bet are very important in the study of finiteness conditions for semigroups. This problem consists in considering conditions which are satisfied...