The delicate and elaborate structure of the fish gill has long fascinated biologists as perhaps the most exquisite example of an external respiratory organ. This chapter summarizes gill anatomy and describes the application of scanning electron microscopy (SEM) to studies of gill form and function. Emphasis is placed on key or unique SEM studies that raise interesting questions to be pursued further, suggest unique methodological applications, or define specific areas that are actively being investigated. The advantages of SEM methods include: ease of sample preparation, three-dimensional perspective, high resolution, and the ability to survey large areas of tissue. X-ray analysis, in conjunction with SEM, offers a quantitative elemental analysis of gill tissue and has considerable potential in physiological as well as toxicological studies. The gill is the primary corridor for molecular exchange between the internal milieu of a fish and its environment. Gills perform myriad seemingly unrelated homeostatic functions that include respiration, osmoregulation, acid-base balance and nitrogen excretion.