  We consider methods based on the topological degree theory to compute
periodic orbits of area preserving maps. Numerical approximations of the
Kronecker integral and the application of Stenger's method allows us to compute
the value of the topological degree in a bounded region of the phase space. If
the topological degree of an appropriate set of equations has a non--zero
value, we know that there exists at least one periodic orbit of a given period
in the given region. We discuss in detail the problems that these methods face,
due to the existence of periodic orbits near the domain's boundary and due to
the discontinuity curves that appear in maps defined on the torus. We use the
characteristic bisection method for actually locating periodic orbits. We apply
this method successfully, both to the standard map, which is a map defined on
the torus, and to the beam--beam map which is a continuous map on the plane.
Specifically we find a large number of periodic orbits of periods up to 40,
which give us a clear picture of the dynamics of both maps.
