The mitochondrial genome is vital for Caenorhabditis elegans metabolism, physiology, and development. The C. elegans mitochondrial DNA is typical of animal mitochondrial genomes in its size and gene content. It is 13,794 nucleotides in length and encodes 36 genes: 2 ribosomal RNAs, 22 transfer RNAs, and 12 protein subunits of the mitochondrial respiratory chain. Although it represents only a small number of genes, an elaborate cellular machinery comprised of over 200 nuclear genes is needed to replicate, transcribe, and maintain the mitochondrial chromosome and to assemble the translation machinery needed to express this dozen proteins. Mitochondrial genetics is peculiar and complex because mitochondrial DNA is maternally inherited and can be present at tens to tens of thousands of copies per cell. The mitochondrial genome content of the developing nematode is developmentally regulated; it increases about 30-fold between the L1 and the adult stages and blocking the increase leads to larval arrest. Energy metabolism is also intimately linked to aging and lifespan determination. The nematode model system offers numerous advantages for understanding the full importance and scope of the mitochondrial genome in animal life.