The inflammatory process in arthropods includes primarily the recruitment of circulating hemocytes to wounds or sites of microbial infections. Melanization, capsule formation and clotting reactions will finally result in the sealing of wounds. In this review we will focus on recent research about hemolymph clotting and melanization reactions, and the recruitment of hemocytes to wounds and infections. We further describe in more detail new knowledge about crustacean hematopoiesis that is crucial for hemocyte recruitment to the site of an infection and there develop an inflammatory response. Moreover, we pay special attention to the gut as an important route of infection in arthropods. Since the gastrointestinal tract provides a first line of defense and regulation of the indigenous bacteria and the intestine often harbors loads of potential pathogenic microorganisms, the integrity of intestinal epithelium and to maintain the correct flora is crucial to animal health.