Cytoplasmic male sterility arises when mitochondrial activities are disrupted that are essential for pollen development. Rearrangements in the mitochondrial genome that create novel open reading frames are strongly correlated with CMS phenotypes in a number of systems. The morphological aberrations which indicate CMS-associated degeneration are frequently restricted to the male sporogenous tissue and a limited number of vegetative tissues. In several cases, this tissue specificity may result from interactions between the mitochondrial genome and nuclear genes that regulate mitochondrial gene expression. A molecular mechanism by which CMS might be caused has not been conclusively demonstrated for any system. Several hypotheses for general mechanisms by which mitochondrial dysfunction might disrupt pollen development are discussed, based on similarities between the novel CMS-associated genes from a number of systems.