Choosing the Right Model

Building Agent-Based Models requires assumptions (as does building any model).The advantage of ABM is the assumptions can be realistic, rather than made in order to solve a calculus-based problem. But what does "realistic" mean? I shall outline the issues of verification of a model (is it working as the modeller wants?) and validation of a model (how close is the model behaviour to some measures of reality?), after talking about the general issue of choosing models.