It is an understatement to say that people are confused about cloud seeding. While it has been called "the crime of the century" and outlawedin Pennsylvania, the governments of the dry, western part of the United States continue to spend tax revenues on cloud seeding to increase water supplies. During the past five years, I have talked with officials responsible for decisions regarding cloud seeding programs in about 15 states of the U.S.A., in a dozen other countries, and in the World Meteorological Organization. Some of the officials involved are experts in atmospheric physics, but most are not. Members of the latter group have expressed agreat need for reliable, concise information about how cloud seeding works and what it does to the weather. This book is addressed to them and to all students of the environmental sciences in general and of the atmospheric sciences in particular who need answers to those questions. The book should be useful to graduates of a university program in any of the physical sciences or engineering, and to upperclassmen in those subjects. A review of basic cloud physics is provided to make the presentation meaningful to persons with only minimal preparation in meteorology. The presentation of statistical methods used in the evaluation of cloud seeding projects presupposes no special knowledge of statistics on the part of the reader.