Bacteria that colonize the intestine, invade intestinal epithelial cells, and/or produce one or more toxins are important etiologies of diarrheal disease in both industrialized and developing countries. The primary focus of this review will be to discuss the toxins produced by human enteric pathogens and how the mechanisms of action of these toxins have been linked to intestinal secretion. To accomplish these goals, the range of criteria utilized to demonstrate pathogenicity of an enteric bacterial toxin and potential mechanisms stimulating net intestinal secretion will be reviewed (Table 1). This will be followed by a discussion of the criteria used to classify a given bacterial enteric toxin. Because of the numerous, often overlapping systems by which bacterial toxins can be classified on the basis of mechanisms, we will first discuss potential enteric toxins grouped by organism. Following this initial recitation, the toxins will be discussed on the basis of potential mechanisms yielding secretion in the intestinal epithelium. Lastly, enteric bacterial toxins acting by more than one potential mechanism to cause intestinal secretion will be discussed. This growing area of investigation, which suggests that a bacterial toxin may subvert multiple cellular processes to yield intestinal epithelial cell secretion, is particularly intriguing. In each case, an attempt has been made to be explicit about the limitations of the data demonstrating pathogenicity and the ability to classify the toxin. Throughout this review, emphasis has been given to data derived from studies of toxins in potentially relevant models of disease, namely, intestinal mucosa in vivo and/or intestinal epithelial cells in vitro.