Habitable planets are likely to be broadly Earth-like in composition, mass, and size. Masses are likely to be within a factor of a few of the Earth's mass. Currently, we do not have sufficiently sensitive techniques to detect Earth-mass planets, except in rare circumstances. It is thus necessary to model the known exoplanetary systems. In particular, we need to establish whether Earth-mass planets could be present in the classical habitable zone (HZ) or whether the giant planets that we know to be present would have gravitationally ejected Earth-mass planets or prevented their formation. We have answered this question by applying computer models to the 152 exoplanetary systems known by 2006 April 18 that are sufficiently well characterized for our analysis. For systems in which there is a giant planet, inside the HZ, which must have arrived there by migration, there are two cases: (1) where the migration of the giant planet across the HZ has not ruled out the existence of Earth-mass planets in the HZ; and (2) where the migration has ruled out existence. For each case, we have determined the proportion of the systems that could contain habitable Earth-mass planets today, and the proportion for which this has been the case for at least the past 1000Myr (excluding any early heavy bombardment). For case 1 we get 60% and 50%, respectively, and for case 2 we get 7% and 7%, respectively.