Bacterial toxins which act on intestinal cells display a great diversity of size, structure and mode of action. Some toxins interact with the cell by transducing a signal across the membrane leading to stimulation of intracellular second messenger (E. coli heat stable enterotoxin), others form pores (C. perfringens enterotoxin, ...) permitting the leakage of cellular components and cell lysis. The most sophisticated toxins comprise at least two functional domains or components, one being a binding domain permitting the internalization into the cell of an enzymatic domain which modifies an intracellular target. The enzymatic modification (ADP-ribosylation, UDP-glucosylation, glycohydrolysis, proteolysis, ...) of a specific target (heterotrimeric G-protein, small G-protein, monomeric actin, ribosomal RNA, ...) alters the cell physiology (increase of ions and water secretion, cytoskeleton rearrangement, protein synthesis inhibition, apoptosis, ...) and tissue organization (modification of barrier permeability, necrosis, ...). The study of bacterial toxins leads to the understanding of the interactions between pathogenic bacteria and their hosts and constitutes also a new approach in cell biology, by facilitating the exploration of certain regulatory pathways such as that controlling actin polymerization.