The stable inheritance of bacterial plasmids is achieved by a number of different mechanisms. Among them are resolution of plasmid oligomers into monomers, active plasmid partitioning into dividing cells and selective killing of plasmid-free segregants. A special focus is given to the last mechanism. It involves a stable toxin and an unstable antidote. The antidotes neutralize their cognate toxins or prevent their synthesis. The different decay rates of the toxins and the antidotes underlie molecular mechanisms of toxin activation in plasmid-free cells. By eliminating of plasmid-free cells from the population of plasmid-bearing ones the toxin-antidote couples therefore act as plasmid addiction systems.