We have examined the hemocytes of three decapod crustaceans (Homarus americanus, Panulirus interruptus, and Loxorhynchus grandis) and propose a classification of these cells based on morphology, cytochemistry, and studies of cell functions. In all species, hyaline cells and granulocytes were identified. Although we have retained the widely used names for these cells, we show that traditional morphological features alone do not accurately differentiate between these categories. Historically, the term hyaline cell refers to hemocytes that contain no or only a few cytoplasmic granules, whereas granulocytes contain abundant granules. However, the size and number of granules in hyaline cells vary greatly between species and therefore are not useful criteria for identifying these cells. Since morphological identification alone is inadequate and misleading, especially with regard to hyaline cells, a combination of morphological, cytochemical and functional methods is necessary to identify decapod hemocytes. Features of hyaline cells include: a higher nucleocytoplasmic ratio than that of granulocytes, the presence of abundant small ({approx}50 nm), round, electron-dense deposits in the cytoplasm, and their accumulation of trypan blue dye prior to cytolysis. Granulocytes do not take up trypan blue or lyse during a 5-min incubation, and they contain prophenoloxidase and hydrolases. Hyaline cells are involved in the initiation of hemolymph coagulation whereas granulocytes are involved in defense against foreign material by phagocytosis and encapsulation. We propose that these criteria be applied to other crustacean species and expect that they will facilitate our understanding of the physiological roles of their hemocytes.