This book is a comprehensive review of the entire lineage of monitor lizards (Varanidae), living and extinct. It fills a gap in our understanding of the relationships and role of environmental and ecological diversity and their effects on evolution and ecology. The evolutionary history of varanids stretches back some 70 million years. These lizards, so often described as "morphologically uniform" differing mainly in size, are incredibly diverse. This variation is comprised of numerous small features adding up to 80-plus distinctly different species. Synthesizing the latest research, plus a half-century of original observation and research by the author, this text presents a wide-ranging account of the broad biology and evolution of a successful group of vertebrates adapted to environments from the harshest deserts to lushest forests and even tropical beaches. Monitors are shown to be far from a homogeneous group, a lineage in which subtle variations have led to profoundly divergent organisms.