The transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator-1 (PGC-1) has been implicated in important metabolic processes. A mouse lacking PGC-1 (PGC1KO) was generated and phenotyped using physiological, molecular, and bioinformatic approaches. PGC1KO mice are generally viable and metabolically healthy. Using systems biology, we identified a general defect in the expression of genes involved in mitochondrial function and, specifically, the electron transport chain. This defect correlated with reduced mitochondrial volume fraction in soleus muscle and heart, but not brown adipose tissue (BAT). Under ambient temperature conditions, PGC-1 ablation was partially compensated by up-regulation of PGC-1 in BAT and white adipose tissue (WAT) that lead to increased thermogenesis, reduced body weight, and reduced fat mass. Despite their decreased fat mass, PGC1KO mice had hypertrophic adipocytes in WAT. The thermogenic role of PGC-1 was identified in thermoneutral and cold-adapted conditions by inadequate responses to norepinephrine injection. Furthermore, PGC1KO hearts showed a blunted chronotropic response to dobutamine stimulation, and isolated soleus muscle fibres from PGC1KO mice have impaired mitochondrial function. Lack of PGC-1 also impaired hepatic lipid metabolism in response to acute high fat dietary loads, resulting in hepatic steatosis and reduced lipoprotein-associated triglyceride and cholesterol content. Altogether, our data suggest that PGC-1 plays a general role in controlling basal mitochondrial function and also participates in tissue-specific adaptive responses during metabolic stress.