Biosynthesis of a functional mitochondrion requires the coordinate expression of genes in both mitochondrial and nuclear DNAs. In yeast, three mitochondrial genes are split and RNA splicing plays a pivotal role in their expression. The recent finding that some introns are capable of self-splicing activity in vitro has permitted analysis of the mechanisms involved in RNA catalysis and may eventually shed light on the evolution of splicing mechanisms in general. Most mitochondrial proteins are encoded by nuclear genes, synthesized in the cytoplasm and imported by the organelle. The availability of cloned genes coding for several constituent subunits of the ubiquinol-cytochrome c reductase, which are imported by mitochondria, has allowed study of selected steps in the addressing of proteins to mitochondria and their intercompartmental sorting within the organelle. Recent developments are discussed.