AbstractThis chapter elaborates on pathogenic processes responsible for the production of symptoms during giardiasis and cryptosporidiosis. To date, research findings indicate that both infections share a number of these processes. Infection appears to cause diarrhoea via a combination of intestinal malabsorption and hypersecretion. Malabsorption and maldigestion mainly result from a loss of total epithelial brush border surface area, and diffuse microvillous shortening is mediated by activated host T lymphocytes. This activation is secondary to Giardia- and Cryptosporidium-induced disruption of epithelial tight junctions, which in turn increases intestinal permeability. Both parasites may breach the epithelial barrier by inducing enterocyte apoptosis. These effects may facilitate the development of other enteric disorders in infected patients, including inflammatory bowel disease, irritable bowel syndrome and allergies, via mechanisms that remain obscure. The regulatory processes of epithelial apoptosis are also discussed, including the role of parasite products as well as host factors. In this context, recent observations indicate that host epithelial nitric oxide responses, as well as a newly discovered glucose-mediated cytoprotective mechanism, may represent effective modulators of the epithelial apoptosis induced by these parasites. This review of the various pathogenic mechanisms of giardiasis and cryptosporidiosis sheds light on potential therapeutic targets that may help control the disease associated with these infections as well as a variety of other enteric disorders.