A quarter century has elapsed since the last monographic review of knowledge about urban climates appeared in book Kratzer (1956). That summarization came at a time when a certain plateau of knowledge had been reached. Essentially, it marked the end of a descriptive, geographic phase that established beyond doubt the fact that urban areas have climates different from their surroundings. Since that time emphasis has been on analytical studies directed toward physical understanding of the rural-urban differences in the atmospheric boundary layer. Attempts to quantify conditions have met with some success. There is certainly a clear understanding of the physical relations that create the climatic differences of urbanized areas. This understanding represents a new plateau of knowledge, deserving summarization, and that is the aim of this book. No attempt has been made here to give comprehensive coverage of the literature. Although some of the earlier classical studies are cited, the emphasis is on the work done during the last decade and a half.