We propose a framework for decision support in a changing world that rests on mechanism-based causal models. A causal model is a self-contained set of simultaneous structural equations representing a decision problem. In our framework, a causal model is instantiated from a mechanism knowledge base, where domain knowledge is stored as mechanisms along with generic non-intervening and intervening actions. A change in the world is modeled as applying an action on the causal model. The framework supports reasoning about the effect of actions to deduce the causal model representing the world brought about by the actions.