The first disorder of mitochondrial function was described by Luft in 1959. Over the ensuing decades, multiple cases of mitochondrial dysfunction were reported, and the term “mitochondrial disorder” arose to describe any defect in the mitochondrial electron transport chain. The sequence of the mitochondrial genome was elucidated in 1981 by Anderson et al., and during the next 20 years, >200 pathogenic point mutations, deletions, insertions, and rearrangements were described. Most of the original cases were adults, and the diagnosis of a mitochondrial disorder in an adult patient became relatively straightforward. Adults present with well-defined “mitochondrial syndromes” and generally carry mitochondrial DNA mutations that are easily identified. Children with mitochondrial disorders are much harder to define. Children are more likely to have a nuclear DNA mutation, whereas the “classic” syndromic findings tend to be absent. This review describes both the varying presentations of mitochondrial disorders and the common laboratory, imaging, and pathologic findings related to children.