Mitochondria are dynamic organelles that undergo cycles of homotypic fusion and fission, which are believed to play an important role in controlling organelle number, subcellular distribution, morphology, and ATP production. In mammals, mitochondrial fusion is essential for embryonic development and is controlled by the mitofusins, Mfn1 and Mfn2. Despite our increasing knowledge about importance of mitofusins, we have almost no understanding of how they function from a mechanistic perspective. Here we review our current molecular understanding of murine mitofusins, and discuss how mitofusins might mediate mitochondrial fusion.