Many membranes in eukaryotic cells are inhomogeneous structures in which various membrane components are nrandomly distributed, forming diverse types of 'domains.' Some membrane domains have long been well kwn, because they are sufficiently large, long-lived, and morphologically well defined to be characterized using classical microscopic and biochemical approaches. However, new techlogies have revealed the presence in membranes of smaller, often highly dynamic 'nadomains' that also play key roles in membrane function. Our current understanding of the diversity, the properties, and the functions of nadomains is still very limited and, in some cases, controversial. Nonetheless, it is clear that many important aspects of membrane biology arise from features of membrane organization that 'play out' on spatial and temporal scales that are only w becoming experimentally accessible in living systems. In this book, we will discuss properties and interactions of membrane molecules that lead to nadomain formation, new and emerging techlogies by which nadomains can be studied, and experimental examples that illustrate both highlights and current limitations of our present kwledge of the properties of membrane nadomains in various cell types.