In this paper Poisson`s equation in two dimensions is studied and different solutions with a BEM are outlined depending on whether the source function is harmonic or not. When the Laplacian of the source function is zero or a constant, the Galerkin vector and the multiple-reciprocity method are applied. When the Laplacian of the source function is no longer zero, the source function is approximated by Lagrange polynomials. This method is then improved by subtracting a parabola from the source function.