This paper deals with the inverse design in the field of photonic crystal (PC)-based devices. Here, an inverse method containing a fast and accurate simulation method integrated with a competent optimization method is presented. Two designs yielded from this conjunction of multiple scattering theory with a genetic algorithm are analyzed. The potential of this approach is illustrated by designing a lens that has a very low F-number (F=0.47) and a conversion ratio of 11:1. We have also designed a coupler device that introduces the light from an optical fiber into a PC-based waveguide with a predicted coupling efficiency that exceeds 87%.