The well-known Casimir effect has a direct analogue in systems near critical or multicritical points. Critical fluctuations in systems confined to finite geometries lead to attractive or repulsive forces between system boundaries. These forces influence the formation of wetting layers of liquid 4He or binary liquid mixtures near critical points in these fluids. With the aid of recently developed versions of the atomic force microscope, these forces appear to be directly measurable. The book contains an introduction to the physics of critical phenomena and reviews the most recent developments in the theory of finite-size scaling. A detailed discussion of the Casimir effect and related questions follows. The analysis of quantitative effects on the specific heat of critical films, the formation of wetting layers, and force measurements finish the presentation. This is perhaps the first book on the critical Casimir effect.