Since the circulating hemocytes are involved in the key physiological functions in insects, knowledge of these haemocytes is necessary to physiologists, toxicologists and biochemists. The present article was prepared to aim at updating the identification of hemocyte types and discussing different terminological and technical difficulties. It focused, also, on most important quantitative parameters of haemogram and discussed various factors influencing their values. In this review, we described, also the origin of circulating hemocytes during embryonic and the postembryonic development as well as intensively reviewed the categorization of hemocyte types in several insect species of different orders. The controversial terminology and technical difficulties of the hemocyte identification were discussed. It was emphasized that none of the individual methods was satisfactory for all cell types within a given insect but a combination of techniques should be used. The present chapter included only on the major quantitative parameters of haemogram, such as the total haemocyte count (THC), blood volume (BV) and mitotic index. It shed some light on the interrelationship between THC and BV. In addition, the endocrine control of THC and BV had been reported. The interrelationships between BV and osmotic pressure as well as between mitotic index and THC had been discussed. Heartbeat rate was reported to depend on different factors but the control mechanisms for the insect heart are not fully elucidated