Governments and other policy makers use long-term planning models to support workforce planning decisions for regulating care markets and to ensure accurate balancing between care supply and demand. Our aim is to understand long-term effects of workforce planning decisions on care markets, in order to enhance health care policy making. We identify 25 key factors that influence care demand and supply based on literature analysis and analysis of a planning model for long term care development currently used in the Netherlands. We depict a model that relates these key factors together, and, ultimately construct a system dynamics model to predict long-term development of specialist care supply and demand. We conclude that long-term developments of care markets are not only determined by these 25 factors but also by dynamic interactions among three markets: (1) the specialist care markets, (2) the personnel markets in hospitals and specialist groups, and (3) the specialist training markets. Planning models must include such interactions to ensure valid long-term predictions of markets and workforce needs.