As California implements the Affordable Care Act (ACA) its leaders require a good understanding of what the likely impacts of the law will be on its residents – and the state’s budget. To help them, Dr. Dylan Roby, director of the Health Economics and Evaluation Program at the UCLA Center for Health Policy Research and an assistant professor in the UCLA Jonathan and Karin Fielding School of Public Health, has partnered with the UC Berkeley Center for Labor Research and Education to create the California Simulation of Insurance Markets (CalSIM), a micro-simulation model that can be used to estimate the impact of various elements of the ACA.