This book deals with the thick, rubbery, gooey, and pasty substances that defy the classical definitions of solids and liquids. These substances are often called "complex fluids," which I define as "substances that flow at modest stresses." By "flow," I mean a smooth deformation on a humanly accessible time scale. A piece of slate or ceramic does not "flow"; it deflects under modest loads, and it fractures under great ones. Peanut butter, on the other hand, "flows" rather smoothly when a child pushes a butter knife across it. Hence peanut butter is a "complex fluid," while a brick is not. Although the packed ices of glaciers "flow" on geological time scales, and metals creep under large loads by defect motion, and "even the mountains flow before the Lord," we mortals usually prefer to call these solids. Clearly, there are ambiguous cases: A piece of window glass seems solid enough at room temperature and ordinary periods of time, but heat it up a bit, or wait a few decades, and its solidity becomes dubious. Reiner A949) expressed well the dilemma: "Strictly defined rheological divisions belong to ideal abstract bodies and not to real materials. If we say that concrete is a liquid, every builder will laugh at us.... If we say that glass is a solid, the theoretical physicists will consider us to be simple and crude." The definition of a "complex fluid" might therefore seem as plastic as the materials it is meant to define! Nevertheless, so many materials fall well within the rough definition given above that "complex fluids" are worthy of a title—and indeed, a book—of their own.