The aim of this book is to present several new theoretical results in connection to the field of Fixed Point Theory. This work is devoted to the framework of generalized metric spaces, in which we have included the usual metric spaces, along with convex metric spaces, b-rectangular metric spaces and cone metric spaces over Banach algebras. Furthermore, we have dealt with single-valued operators and at the same time we have treated the case of multi-valued operators. The original results for each chapter are briefly presented in the introduction, which aids to the presentation of this material. Finally, we emphasize the fact that our results can help the readers who aim to study some recent trends in the field of Nonlinear Analysis.