A substantial amount of research on Boundary Elements has taken place since publication of the first Volume of this series. Most of the new work has concentrated on the solution of non-linear and time dependent problems and the development of numerical techniques to increase the efficiency of the method. Chapter 1 of this Volume deals with the solution of non-linear potential problems, for which the diffusivity coefficient is a function of the potential and the boundary conditions are also non-linear. The recent research reported here opens the way for the solution of a: large range of non-homogeneous problems by using a simple transformation which linearizes the governing equations and consequently does not require the use of internal cells. Chapter 2 summarizes the main integral equations for the solution of two-and three dimensional scalar wave propagation problems. This is a type of problem that is well suited to boundary elements but generally gives poor results when solved using finite elements. The problem of fracture mechanics is studied in Chapter 3, where the ad vantages of using boundary integral equations are demonstrated. One of the most interesting features of BEM i~ the possibility of describing the problem only as a function of the boundary unknowns, even in the presence of body, centrifugal and temperature induced forces. Chapter 4 explains how this can be done for two-and three-dimensional elastostatic problems.