Mitochondria play an essential role in the ability of brown fat to generate heat, and the PGC-1 coactivators control several aspects of mitochondrial biogenesis. To investigate their specific roles in brown fat cells, we generated immortal preadipocyte lines from the brown adipose tissue of mice lacking PGC-1α. We could then efficiently knockdown PGC-1β expression by shRNA expression. Loss of PGC-1α did not alter brown fat differentiation but severely reduced the induction of thermogenic genes. Cells deficient in either PGC-1α or PGC-1β coactivators showed a small decrease in the differentiation-dependant program of mitochondrial biogenesis and respiration; however, this increase in mitochondrial number and function was totally abolished during brown fat differentiation when both PGC-1α and PGC-1β were deficient. These data show that PGC-1α is essential for brown fat thermogenesis but not brown fat differentiation, and the PGC-1 coactivators play an absolutely essential but complementary function in differentiation-induced mitochondrial biogenesis.