We use statistics on the nuclear magnitudes of Jupiter Family comets to derive the number of km-sized members with perihelion distances less than 2 AU. When coupled with impact rates per comet taken from Levison et al. (2000), and with our estimate of the number of dormant comets in similar orbits yielding a dormant/active ratio of about 2, we find a terrestrial impact rate of about 1×10-6 per year. This is already significant (~20 - 50%) compared with the total estimated impact rate by km-sized bodies from cratering statistics, and we estimate that further contributions by Halley-type and long-period comets are also substantial. Thus, in broad agreement with a number of earlier investigators, we find that comets yield a large, perhaps dominant, contribution to km-sized terrestrial impactors.