Using as a theme the encounter between protozoan parasites and macrophages, this volume brings together cell biologists, immunologists and protozoologists to review current developments in this broad and dynamic research area. Discussed are ways protozoans establish their intracellular niche, how they activate macrophage effector functions, what these functions are, and means by which several protozoans subvert macrophage activity. What emerges is a picture of the macrophage as a key cell type in the host response to protozoan infection. How these cells respond, and how their responses can be subverted, are likely to be critical determinants in the outcome of protozoan infection. Millions of lives are lost every year to protozoan infections, most importantly those caused by Plasmodium, but also Trypanosoma and Leishmania spp. Other protozoans, such as Toxoplasma, are so exquisitely adapted to their host that they rarely cause disease, except during host immunodeficiency. We hope that by working towards an understanding of the biology of protozoans in macrophages we will ultimately be capable of treating and preventing disease and mortality caused by this major class of microbial pathogens.