Plant and animal mitochondrial genomes, although quite distinct in size, structure, expression and evolutionary dynamics both may exhibit the state of heteroplasmy--the presence of more than one type of mitochondrial genome in an organism. This review is focused on heteroplasmy in plants, but we also highlight the most striking similarities and differences between plant and animal heteroplasmy. First we summarize the information on heteroplasmy generation and methods of its detection. Then we describe examples of quantitative changes in heteroplasmic populations of mitochondrial DNA (mtDNA) and consequences of such events. We also summarize the current knowledge about transmission and somatic segregation of heteroplasmy in plants and animals. Finally, factors which influence the stoichiometry of heteroplasmic mtDNA variants are discussed. Despite the apparent differences between the plant and animal heteroplasmy, the observed similarities allow one to conclude that this condition must play an important role in the mitochondrial biology of living organisms.