Statistical mechanics is the branch of science that tries to account for the bulk properties of matter in terms of its microscopic constituents. The behaviour of the latter, which might for example be atoms or molecules, is of course very well known and understood in pnnciple, being governed by Newtonian dynamics and electromagnetism. The constituent particles are usually so numerous, however, that their sheer numbers render it quite impossible in practice to follow all their individual motions and on this basis to arrive at an understanding of the bulk properties of the piece of material. A statistical approach to the problem is unavoidable.