  We consider the general spatial three body problem and study the dynamics of
planetary systems consisting of a star and two planets which evolve into 2/1
mean motion resonance and into inclined orbits. Our study is focused on the
periodic orbits of the system given in a suitable rotating frame. The stability
of periodic orbits characterize the evolution of any planetary system with
initial conditions in their vicinity. Stable periodic orbits are associated
with long term regular evolution, while unstable periodic orbits are surrounded
by regions of chaotic motion. We compute many families of symmetric periodic
orbits by applying two schemes of analytical continuation. In the first scheme,
we start from the 2/1 (or 1/2) resonant periodic orbits of the restricted
problem and in the second scheme, we start from vertical critical periodic
orbits of the general planar problem. Most of the periodic orbits are unstable,
but many stable periodic orbits have been, also, found with mutual inclination
up to $50^\circ$ - $60^\circ$, which may be related with the existence of real
planetary systems.
