This book quickly introduces beginners to general group theory and then focuses on three main themes: * finite group theory, including sporadic groups; * combinatorial and geometric group theory, including the Bass–Serre theory of groups acting on trees; * the theory of train tracks by Bestvina and Handel for automorphisms of free groups. With its many examples, exercises, and full solutions to selected exercises, this text provides a gentle introduction that is ideal for self-study and an excellent preparation for applications. A distinguished feature of the presentation is that algebraic and geometric techniques are balanced. The beautiful theory of train tracks is illustrated by two nontrivial examples.