Control engineers have used dynamic simulations for decades to study process control concepts, investigate loop interactions and design control strategies. This activity has typically been the domain of the expert engineer. A typical model of a single distillation column could take six months to build and test. Improved simulation technology coupled with improved computer hardware and software techniques has begun to change the level of expertise needed to build models and run simulations. With this new technology we can build the entire refining system for a chemical plant in two weeks. In this paper, I will discuss the need and uses for dynamic simulation, outline the current trends in the industry and provide case studies that have resulted in $MM of improvements in plant operability. I will discuss some of the main issues concerning dynamic modeling, which include basic data needs and model complexity.