GRIN is a package to solve elliptic problems in two dimensions using Green's functions. The advantage of the Green's function formalism compared to a finite element approach for instance, is that one is left with a lower dimensional problem, namely the one of computing line integral in two-dimensions. Because the Green's functions can be chosen so as to vanish at infinity, this formalism is particularly applicable to problems where the domain is large or extends to infinity. Another typical application of the Green's function formalism is when one is only interested in determining the field on selected contours.