Cell fusions are important to fertilization, placentation, development of skeletal muscleand bone, calcium homeostasis and the immune defence system. Additionally, cellfusions participate in tissue repair and may be important to cancer development, progression and therapy. A large number of factors regulate cell fusions, including receptorsand ligands, membrane domain organizing proteins, proteases, signaling molecules andfusogenic proteins that bring membranes close together. In the present volume, we interrogate what mechanisms that may be important to cell fusions in mammals and what mechanisms that may be shared with lower organisms and viruses. Additionally, the importance of cell fusions to the pathogenesis as well as treatment of diseases is addressed.