Escherichia coli K-12 contains at least 36 toxin genes, the expression of which causes growth inhibition and eventual death. These toxins are usually co-expressed with their cognate antitoxins in operons called toxin-antitoxin (TA) modules. Under normal growth conditions, toxins and antitoxins form stable complexes. However, stress-induced proteases preferentially eliminate unstable antitoxins, releasing free toxins to inhibit various cellular functions. TA systems have important roles in the physiology of cells in their natural habitats, including functions in biofilm formation and multidrug resistance. In this Review, we describe these TA systems in light of their functions and roles in the regulation of cell growth and death.