Thirty five years have passed since the first lithography process models were presented, and since that time there has been remarkable progress in the predictive power, performance, and applicability of these models in addressing many different challenges within the semiconductor industry. The impact has been profound, and this paper will attempt to highlight some of the key contributions which have been made, particularly as patterning simulation has moved beyond the realm of process development to full chip production enablement. In addition, this paper will outline the new process simulation challenges which emerge as the industry approaches sub-0.25 k1 patterning. These challenges lie principally in driving towards ever improved accuracy for an expanding set of processes and failure modes, while maintaining or improving full chip data preparation cycle times.