Spatial structures often embody generative systems. Both analog (physical modeling), as well as computational methods, have been used to explore the range of design possibilities. Whereas many of the favored physical modeling techniques, such as soap films or catenary nets, inherently generate forms based on certain performative properties, many of the parametric form generating computational methods derive form based solely on geometry, detached from physical performance. ParaGen has been developed as a tool to explore parametric geometry based on aspects of performance. Within the cyclic structure of a genetic algorithm, it incorporates parametric geometry generation, simulation for performance evaluation, and the ability to sort and compare a wide range of solutions based on single or multiple objectives. The results can be visually compared by teams of designers across a graphic web interface which includes the potential for human interaction in parent selection and breeding of further designs. The result is a tool which allows the exploration of the generative design space based on performance as well as visual criteria.