Relativistic thermal field theory gives the theoretical framework for the description of the quark-gluon plasma, a new state of matter whose existence is predicted by lattice gauge theory and which might be produced in collisions of ultrarelativistic heavy ions. It is also relevant for the description of some astrophysical processes and for the history of the early Universe. The first part of this book is a self-contained introduction to the subject, which assumes that the reader has some familiarity with quantum field theory and statistical mechanics. Topics include the path integral approach, the realand imaginary-time formalisms, fermion fields and gauge fields at finite temperature. Useful techniques such as the evaluation of frequency sums and the use of cutting rules are illustrated by various examples. The second part of the book is devoted to recent developments. It gives a detailed account of collective excitations, bosonic and fermionic, and it illustrates how these give rise to energy scales which imply a reorganization of perturbation theory. The relation with kinetic theory is also explained. Applications to processes which occur in heavy ion collisions and in astrophysics are worked out in detail. The reader will find exercises and a guide to the literature at the end of each chapter. . The book will be of interest to theoreticians working in elementary particle physics, nuclear physics and astrophysics.