This book offers an introduction to General Relativity and its mathematical tools, together with an introduction to relativistic and scalar-tensor cosmologies. Part I deals with Tensor Calculus. Part II introduces General Relativity Theory, while Part III deals with Relativistic Cosmology. In Part IV we work Scalar-Tensor theories, concentrating in Cosmological Models. In the last chapters, the cosmological models presented, become more and more sophisticated, including some new cases, never published elsewhere, in which all fundamental "constants" are made to vary, with the age of the Universe, namely, the gravitational, the cosmological, the coupling Brans-Dicke "constants", the speed of light, Planck's "fine -structure "constant" alpha" etc. This is a mathematical cosmology textbook that may lead undergraduates, and graduate students, to one of the frontiers of research, while keeping the prerequisites to a minimum, because most of the theory in the book requires only prior knowledge of Calculus and a University Physics course.