Estrogen-related receptor alpha (ERRalpha) is one of the first orphan nuclear receptors to be identified, yet its physiological functions are still unclear. We show here that ERRalpha is an effector of the transcriptional coactivator PGC-1alpha [peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha], and that it regulates the expression of genes involved in oxidative phosphorylation and mitochondrial biogenesis. Inhibition of ERRalpha compromises the ability of PGC-1alpha to induce the expression of genes encoding mitochondrial proteins and to increase mitochondrial DNA content. A constitutively active form of ERRalpha is sufficient to elicit both responses. ERRalpha binding sites are present in the transcriptional control regions of ERRalpha/PGC-1alpha-induced genes and contribute to the transcriptional response to PGC-1alpha. The ERRalpha-regulated genes described here have been reported to be expressed at reduced levels in humans that are insulin-resistant. Thus, changes in ERRalpha activity could be linked to pathological changes in metabolic disease, such as diabetes.