Knowledge of arthropod immune mechanisms has been based to a large extent on studies of non-medically important arthropod-microbial interactions. Investigations involving arthropods and their immune responses against helminth and protozoan parasites they transmit to vertebrate hosts are relatively limited. Data available strongly suggest that effective responses against these parasites are hemocyte mediated reactions, with blood cells involved in parasite recognition and effector mechanisms. It also is apparent that parasites are very successful in evading immune destruction in compatible arthropodparasite associations by avoiding immune detection and/or by actively inhibiting the immune processes. However, an understanding of immune evasion mechanisms operating on behalf of the parasite is dependent on a more thorough understanding of parasite recognition, signal transduction, and effector mechanisms involved in arthropod immunity. Limited data are available on hemocyte recognition events, and nothing is known about the transduction processes whereby cells convert extracellular signals to intracellular messages that activate effector mechanisms. Any significant progress in these important areas of research will be limited until successful in vitro hemocyte cultures are developed. Convincing data are available from a variety of arthropod-parasite systems that phenol oxidases play a major role in effector mechanisms of the immune response. However, critical biochemical studies are needed to clarify their substrate specificities and the uniqueness of these enzymes. It is essential that we identify specific substrates involved in effector mechanisms, and their potential storage forms, if we are to begin to understand immune processes in arthropods. The sensitivity of HPLC with electrochemical detection provides a valuable tool for these investigations. The next several years should prove exciting in regard to our understanding of molecular/biochemical processes of arthropod immunity.