A suite of “desirable features” for Environmental Decision Support Systems (EDSS) is proposed by identifying the general attributes of environmental systems which are of importance to modelling and simulation, and the different categories of users of EDSSs. With these features as a guide, a review and discussion of the approaches to delivering Environmental Decision Support Systems is presented. The two most efficient approaches are: (i) the use of modelling and simulation software tools, and (ii) the use of model integration and re-use techniques. A review of the currently available software tools for environmental modelling and simulation is therefore presented, and an overview of the current research activities in model integration and re-use is provided. Numerous existing EDSS are used as examples of the different approaches throughout the review. The review highlights the advantages and disadvantages of the two main approaches to EDSS delivery, and subsequently discusses the role emerging information technologies may play in the future delivery of EDSS.The paper concludes that while the available software for modelling and simulation is very advanced, some of the desirable features of EDSS (such as representation of spatial data and provision of expert help) cannot be easily delivered using such software. While the research activities in model integration and re-use are making real advances, especially in delivering those features which cannot currently be provided using modelling and simulation software, this is not occurring in any coordinated or synergistic manner. The ultimate goal should be to provide a fully general EDSS development platform which would allow system analysts to develop EDSS for any environmental domain complete with all desirable features. This is still considered to be very much a long-term ambition.