Nowadays, computers are ubiquitous. They can be found in uncountable situations and applications, from the traditional personal computers to smartphones and embedded devices for industrial automation. The latter are the basis of a new digital revolution that is allowing, among other advances, smart cities, self-driving cars and the Internet of Things (IoT). The aim of this book is presenting the techniques involved in developing current computers from three points of view: performance improvement, operating system support and virtualization support. For this, the contents are organized in four chapters: introduction to computer architecture, CPU, memory hierarchy and input/output system. This book is oriented to serving as bibliography in university courses about Computer Architecture, within the studies of Computer and Software Engineering, where a basic understanding of the internals of computers is assumed.