This is an elementary introduction to the representation theory of real
and complex matrix groups. The text is written for students in
mathematics and physics who have a good knowledge of
differential/integral calculus and linear algebra and are familiar with
basic facts from algebra, number theory and complex analysis. The goal
is to present the fundamental concepts of representation theory, to
describe the connection between them, and to explain some of their
background. The focus is on groups which are of particular interest for
applications in physics and number theory (e.g. Gell-Mann's eightfold
way and theta functions, automorphic forms). The reader finds a large
variety of examples which are presented in detail and from different
points of view. The examples motivate the general theory well covered
already by the existing literature. Hence for complete proofs of most of
the essential statements and theorems the reader is often referred to
the standard sources. Plenty of exercises are included in the text. Some
of these exercises and/or omitted proofs may give a starting point for a
bachelor thesis and further studies in a master program.