Though the mitochondrion was initially identified as a key organelle essentially required for energy production and oxidative metabolism, there is considerable evidence that mitochondria are intimately involved in regulating vital cellular processes, such as programmed cell death, proliferation and autophagy. Discovery of mitochondrial “shaping proteins” (Dynamin-related protein (Drp), mitofusins (Mfn) etc.) has revealed that mitochondria are highly dynamic organelles continually changing morphology by fission and fusion processes. Several human pathologies, including cancer, Parkinson’s disease, Alzheimer’s disease and cardiovascular diseases, have been linked to abnormalities in proteins that govern mitochondrial fission or fusion respectively. Notably, in the context of the heart, defects in mitochondrial dynamics resulting in too many fused and/or fragmented mitochondria have been associated with impaired cardiac development, autophagy, and contractile dysfunction. Understanding the mechanisms that govern mitochondrial fission/fusion is paramount in developing new treatment strategies for human diseases in which defects in fission or fusion is the primary underlying defect. Here, we provide a comprehensive overview of the cellular targets and molecular signaling pathways that govern mitochondrial dynamics under normal and disease conditions.  (Circ J 2014; 78: 803–810)