Knowledge of the size distribution of comets and asteroids can be obtained from two sources. First the planet Earth can be regarded as a detector and the diameter distribution of Earth craters used to assess the mass distribution of the incident bodies. Secondly observations from Earth of the characteristics and orbits of comets and asteroids can lead to their collision probabilities and mass distributions. Simplistically it can be stated that the Earth craters have been produced by small incident objects whereas the comets and asteroids that are easily seen are the large ones. This paper seeks to relate these diverse sources of information.