Humans aren't the only vertebrates susceptible to the acute gastroenteritis caused by the protozoan Cryptosporidium. Abdominal pain and diarrhoea, like those caused by a choleric infection, are classic symptoms of this illness. Parasite infections in immunocompromised hosts are often protracted and deadly. This is why cryptosporidiosis is among the most dangerous opportunistic illnesses for people with AIDS. Establishing sensitive and specific diagnostic tests for epidemiological surveillance and morbidity reduction is the best strategy for containing the infection in these patients. Here, we provide a high-level overview of Cryptosporidium infection, with an emphasis on the diagnostic methods now in use to identify cases of cryptosporidiosis. Also covered are the current molecular methods for detection, as well as developments in new diagnostics for cryptosporidiosis.