Domain decomposition methods are designed to allow the effective numerical solution of partial differential equations on parallel computer architectures. They comprise a relatively new field of study but have already found important applications in many branches of physics and engineering. In this book the authors illustrate the basic mathematical concepts behind domain decomposition, looking at a large variety of boundary value problems. Contents include symmetric elliptic equations, advection-diffusion equations, the elasticity problem, the Stokes problem for incompressible and compressible fluids, the time-harmonic Maxwell equations, parabolic and hyperbolic equations, and suitable couplings of heterogeneous equations.