Some new material has been added to this second edition, whilst leaving the organization of the rest of the book (chapters 1 to 12) unchanged. The new chapters aim to illustrate the basic ideas in three rather distinct and (almost) independent ways. Chapter 13 gives a discussion of chemical thermodynamics, including something about chemical equilibrium. Chapter 14 explores how some interacting systems can still be treated by a simple statistical approach, and Chapter 15 looks at two interesting applications of statistical physics, namely superfluids and astrophysics. The book should be useful for university courses of various lengths and types. Several examples follow: Basic general course for physics undergraduates (20 to 25 lectures): most of Chapters 1 to 12, omitting any of Chapters 7, 10, 11 and 12 if time is short; (2) Short introductory course on statistical ideas (about 10 lectures): Chapters 1, 2 and 3 possibly with material added from Chapters 10 and 11; (3) Following (2), a further short course on statisics of gases (15 lectures); Chapters 4-6 and 8-9, with additional material available from Chapter 14 and 15.2 ; (4) For chemical physics (20 lectures): Chapters 1-7 and 10-13; (5) As an introduction to condensed matter physics (20 lectures): Chapters l-6, 8-12, 14, 15.1.