The first-order climate history of the Phanerozoic eon is now known, and it reveals that globally warm intervals without major icecaps represent approximately 75% of the last 540 million years (Frakes, 1979; Fischer, 1982; Crowley, Chapter 14). Study of these greenhouse climates is taking on increased importance in light of climatic warming predicted to result from the doubling of pre-industrial CO2 concentrations over the next century (Broecker, 1997). In spite of the prevalence of warm climates in earth history and the potential practical significance of understanding them, the fundamental causes, nature, and mechanics of warm climates are still poorly understood. We have assembled this book as a way of reviewing what is known about the causes and consequences of globally warm climates, of demonstrating current directions of research on warm climates, and of outlining the central problems that remain unresolved. In serving these goals the chapters present new research on a number of different warm climate intervals from the early Paleozoic to the early Cenozoic. The chapters also integrate a range of approaches, from paleoclimate simulation with atmospheric and oceanic general circulation models, to paleoclimate reconstruction from paleontological and geochemical data, to refinement of paleogeography, to the study of the effects of climate change on marine and terrestrial organisms.