Membrane fusion is ubiquitous in life. Fusion of biological membranes is mediated by specialized fusion proteins anchored to the bilayers destined to fuse. Here we describe these proteins as being instrumental in viral, intracellular and developmental fusion. Next, we review experimental and theoretical evidence that points to fusion in the different systems as following a common 'fusion through hemifusion' pathway. We also focus on the structure and dynamics of the transmembrane segment that anchors the fusion proteins to the bilayer, and its role in driving fusion. In particular, we highlight the influence of this single segment on the surrounding membrane lipids and on the overall shape of the membrane along the way to fusion.