The notion that microdomains enriched in certain specialized lipids exist in membranes has been both attractive and controversial since it was first proposed that such domains, termed rafts, might act as apical sorting devices in epithelial cells. The observation that certain lipids are not extractable in cold nonionic detergent supports the raft concept, but the nature of the in vivo correlate of such detergent-resistant membranes remains enigmatic. In principle, microscopy should be able to determine whether the postulated rafts exist. This article focuses on recent microscopy experiments addressing this question. Several, but not all, results support the raft concept, but further definition of the structure, dynamics and function of lipid domains in various biological contexts is urgently required.