The aim of this work is to analyse the heterogeneous effect of Science and Technology Parks (STPs) on firms' innovation outcomes, contingent on firms' size and innovation effort. Despite the worldwide diffusion of STPs and the increasing literature aimed at analyzing their effect on tenants' performance, empirical evidence on the heterogeneous effect of STPs location on different firms is very scarce. We use information for a representative sample of 39,722 Spanish firms, 653 of them located on 22 of the 25 official Spanish STP. Results show, on the one hand, that firm size is negatively related to an STP location effect and, on the other, that only a small amount of internal innovation effort is required to achieve a very high return from park location.However, firms without innovation efforts do not benefit from a park location. Finally, as internal innovation efforts increase, the park effect reduces, but is still at a high level.