Mitochondrial disease is an increasingly recognized but widely heterogeneous group of multisystemic disorders that commonly involve severe neurodevelopmental manifestations in childhood. This review explores the presentation, genetic basis, and diagnostic evaluation of primary mitochondrial disease. Emphasis is placed on neurodevelopmental findings that may be encountered by a Developmental Pediatrician that should provoke consideration of a mitochondrial disorder. The inheritance patterns and mechanisms by which mutations in genes located in either the nuclear or mitochondrial genomes can cause mitochondrial diseases are discussed. A general overview of the current diagnostic evaluation that can be readily initiated by the Developmental Pediatrician is provided, along with a summary of currently available treatment options.