Long-term prediction of sediment transport and of morphological behaviour in the coastal zone, in response to human interference or to change in environmental conditions (collectively global change) is an increasingly important issue in coastal zone management, especially in relation to the needs for environmentally compatible development. Having in mind those aspects of the response of the coastal system related to long term dynamics of coastal morphology, the paper briefly describes possible approaches to environmental modelling, particularly the modelling of coasts in a typical context of poor experimental information and process knowledge. These approaches will be of help in the impact and vulnerability assessment required for coastal zone management. Reference will mainly be made to long term modelling activities currently performed in the context of the MAST (Marine Science and Technology Program) morphodynamic project on coastal morphology and to possible approaches to "qualitative" modelling which may be used to define tendencies of evolution. lt is argued that significant progress in long-term modelling can be made by adopting an appropriate conceptual framework, particularly a top-down approach. This involves formalizing knowledge and experience and integrating data and available models.