Fusion controls mitochondrial morphology and is important for normal mitochondrial function, including roles in respiration, development, and apoptosis. Key components of the mitochondrial fusion machinery have been identified, allowing an initial dissection of its molecular mechanism. Outer and inner membrane fusion events are coordinately coupled but are mechanistically distinct. Mitofusins are mitochondrial GTPases that likely mediate outer membrane fusion. The dynamin-related protein OPA1/Mgm1p is required for inner membrane fusion and maintenance of normal cristae structure. We highlight recent findings that have advanced our understanding of the mechanism, function, and regulation of mitochondrial fusion.