In this thesis, the two-dimensional Sign-Gordon equation is solved using the boundary element method. Since the Sign-Gordon equation is a state of the wave equation, first the method of modeling and solving the one-dimensional wave equation is explained, and then the exact solution of the two-dimensional wave equation in rectangular areas is discussed. The basic concepts used in the boundary element method, such as the Green theorem, the basic solution of the Laplace equation, and integration on a two-dimensional curve, are described, which can be very instructive for those interested in the boundary element method.