  This paper is a review of the dynamics of a system of planets. It includes
the study of averaged equations in both non-resonant and resonant systems and
shows the great deal of situations in which the angle between the two
semi-major axes oscillates around a constant value. It introduces the
Hamiltonian equations of the $N$-planet problem and Poincar\'e's reduction of
them to $3N$ degrees of freedom with a detailed discussion of the
non-osculating ``canonical'' heliocentric Keplerian elements that should be
used with Poincar\'e relative canonical variables. It also includes Beaug\'e's
approximation to expand the disturbing function in the exoplanetary case where
masses and eccentricities are large. The paper is concluded with a discussion
of systems captured into resonance and their evolution to symmetric and
asymmetric stationary solutions with apsidal corotation.
