  We present conclusions from a large number of N-body simulations of the giant
impact phase of terrestrial planet formation. We focus on new results obtained
from the recently proposed Grand Tack model, which couples the gas-driven
migration of giant planets to the accretion of the terrestrial planets. The
giant impact phase follows the oligarchic growth phase, which builds a bi-modal
mass distribution within the disc of embryos and planetesimals. By varying the
ratio of the total mass in the embryo population to the total mass in the
planetesimal population and the mass of the individual embryos, we explore how
different disc conditions control the final planets. The total mass ratio of
embryos to planetesimals controls the timing of the last giant (Moon forming)
impact and its violence. The initial embryo mass sets the size of the lunar
impactor and the growth rate of Mars. After comparing our simulated outcomes
with the actual orbits of the terrestrial planets (angular momentum deficit,
mass concentration) and taking into account independent geochemical constraints
on the mass accreted by the Earth after the Moon forming event and on the
timescale for the growth of Mars, we conclude that the protoplanetary disc at
the beginning of the giant impact phase must have had most of its mass in
Mars-sized embryos and only a small fraction of the total disc mass in the
planetesimal population. From this, we infer that the Moon forming event
occurred between $\sim$60 and $\sim$130 My after the formation of the first
solids, and was caused most likely by an object with a mass similar to that of
Mars.
