The present configuration of the planets is presumably relatively stable over 1010 orbits or so - otherwise we would not be here - but understanding the long-term orbital evolution of the planets is still central to many issues of solar system dynamics. For example: Why are there no small bodies orbiting between the planets except in the asteroid belt? Why are the planets so regularly spaced (Bode's law)? How has the Earth's orbit evolved throughout its history? We now have direct N-body integrations of the planetary orbits for 100 Myr, together with approximate analyses valid for much longer times. These show that the solar system is chaotic and suggest that it is in a state of marginal stability, slowly evolving towards more stable states by ejecting planets.