The major function of mitochondria is production and supply of cellular energy. Mitochondria are highly dynamic organelles undergoing frequent shape changes via fission and fusion. Many studies have elucidated the molecular components mediating fission and fusion and their regulatory mechanisms, and mitochondrial shape change is now recognized as an essential cellular process that is closely associated with functional states of mitochondria. This review updates the recent advancements in fission and fusion mechanisms, and discusses the bi-directional relationship between mitochondrial morphology and energetic states in physio-pathological settings.