Surface science and colloid science are preeminently experimental subjects. They constitute complementary aspects of a field which has been notably active since World War II; there is every reason to expect that the level of activity will continue to rise in the coming decades, so it is timely to review certain experimental methods of surface and colloid science as they exist, and to evaluate and refine those methods. This volume, and others that will follow, are principally concerned with experimental methods. The working scientist needs access to the latest techniques, of course. He also needs to learn of the potentialities of recently developed techniques which he may not have been aware of. Equally important, or perhaps even more so, he needs to learn of the pitfalls of existing methods. One might say, wistfully, that it would be nice to be able to pick up somebody's description of a new piece of apparatus, to go into the laboratory, to build it, and to have it work, the first time! There is, however, a serious problem of the interaction between the experiment per se and the theory for which the experiment is designed. Very often, this interaction renders problematic the interpretation of "direct" observations. An example, from experience of the senior editor of this volume, is the question of contact angle hysteresis. (See Chapters 1 and 2.