Thelen et al. not only offer an important new theoretical account of the Stage 4 object permanence error but provide the foundation of a new theory of cognitive development that is grounded in action. The success of dynamic field theory as a more general account of cognitive functioning, however, will depend on the degree to which it can model more generative capacities that are not limited to simple choice situations. Imitation and problem solving are suggested as two capacities that might be productively modeled within their approach.