Close encounters of black holes

Processes involving close encounters of black holes, like their mutual scattering or merging, can be analytically approached by Einstein's equations without additional matter Some characteristic features of these equations are reviewed and set in relation to Newtonian concepts. The initial-value problem is explained and techniques are presented that allow the explicit construction of multi-black-hole initial data. Some physical properties of these data are discussed and, finally, a perspective on recent developments is given.