Toxin–antitoxin (TA) systems are small genetic elements found on plasmids or chromosomes of countless bacteria, archaea, and possibly also unicellular fungi. Under normal growth conditions, the activity of the toxin protein or its translation is counteracted by an antitoxin protein or noncoding RNA. Five types of TA systems have been proposed that differ markedly in their genetic architectures and modes of activity control. Subtle regulatory properties, frequently responsive to environmental cues, impact the behavior of TA systems. Typically, stress conditions result in the degradation or depletion of the antitoxin. Unleashed toxin proteins impede or alter cellular processes including translation, DNA replication, or ATP or cell wall synthesis. TA toxin activity can then result in cell death or in the formation of drug-tolerant persister cells. The versatile properties of TA systems have also been exploited in biotechnology and may aid in combating infectious diseases.