Intestinal parasites remain extremely common worldwide. In developing countries, intestinal protozoans are important causes of childhood diarrhea. Cryptosporidiosis is a common cause of chronic diarrhea in patients with AIDS. With the advent of current active antiretroviral therapy the incidence of cryptosporidiosis in AIDS has decreased. By contrast, Cryptosporidium, Cyclospora, and Giardia outbreaks continue to be associated with contamination of food or water. The intestinal helminths Ascaris, hookworm, and Trichuris each infects over a thousand million people. While most of those infected experience only minor symptoms, recent data highlight subtle effects of parasitism on cognitive function and nutrition. Efforts at disease control in developing countries are increasingly focused on mass chemotherapy.