After suggesting why environmental planning seems reluctant to do so, this paper presents a testable hypothesis about how people make planning decisions. Our hypothesis is that no matter what planning problem people address, they always use a common set of criteria in order to evaluate alternative options. Also, the priorities that they place on these criteria remain fairly constant, both through time and across problem domain. We then describe some software that makes our hypothesis operational, and we report on an experiment that used our software to test whether or not our hypothesis is valid. Despite the small samples of people involved, nothing was found to suggest that our hypothesis is false, and so it appears to have considerable potential — subject, of course, to more rigorous testing in the future. Moreover, it has utility right now, as explained in the final section. This describes how our software improved the quality of decisions within a practical, case study, planning situation.