Crustaceans have been shown to possess a primitive immune system that relies mainly on phagocytosis, encapsulation and agglutination. Haemocytes play important roles in crustacean immune responses. It is generally agreed that haematopoietic tissue (HPT) and haematopoietic stem cells are responsible for the production and supply of haemocytes. Most crustacean mature haemocytes were classified into three types by using light microscopy in the past few years. Recently, the monoclonal antibody technique has been used in identifying the types of some crustacean haemocytes, which was proved to be more accurate than classical methods. The cytochemical characteristics and function of different haemocyte subpopulations show significantly difference, suggesting a role for these cells in particular functions. It is demonstrated that total haemocyte counts (THC) changed obviously in crustacean immune response. Generally, the haemocytes defense action in crustaceans relies mainly on phagocytosis when the pathogen is very small, and the microorganism was killed in haemocytes. When a microorganism or parasite is bigger than 10 μm, encapsulation and agglutination played a more important role. Moreover, the immune response of crustacean is very complex, during which cell co-operation and communication, and immune factors interaction are necessary. The study on the structure of HPT and the production mechanism of haemocytes, the establishment of classification standard of haemocyte subpopulations, the function and interaction of immune factors should be the main focus in the future.