  We review previously published and newly obtained crater size-frequency
distributions in the inner solar system. These data indicate that the Moon and
the terrestrial planets have been bombarded by two populations of objects.
Population 1, dominating at early times, had nearly the same size distribution
as the present-day asteroid belt, and produced the heavily cratered surfaces
with a complex, multi-sloped crater size-frequency distribution. Population 2,
dominating since about 3.8-3.7 Ga, has the same size distribution as near-Earth
objects (NEOs), had a much lower impact flux, and produced a crater size
distribution characterized by a differential -3 single-slope power law in the
crater diameter range 0.02 km to 100 km. Taken together with the results from a
large body of work on age-dating of lunar and meteorite samples and theoretical
work in solar system dynamics, a plausible interpretation of these data is as
follows. The NEO population is the source of Population 2 and it has been in
near-steady state over the past ~3.7-3.8 gigayears; these objects are derived
from the main asteroid belt by size-dependent non-gravitational effects that
favor the ejection of smaller asteroids. However, Population 1 were main belt
asteroids ejected from their source region in a size-independent manner,
possibly by means of gravitational resonance sweeping during giant planet orbit
migration; this caused the so-called Late Heavy Bombardment (LHB). The LHB
began some time before ~3.9 Ga, peaked and declined rapidly over the next ~100
to 300 megayears, and possibly more slowly from about 3.8-3.7 Ga to ~2 Ga. A
third crater population (Population S) consists of secondary impact craters
that can dominate the cratering record at small diameters.
