Spacial decision support systems (SDSS) are popular tools in decision-making processes. Despite their popularity there are many systema that are never or hardly used. The present study searches for explanations or reasons for success or failure of such tools by comparing five representative examples of SDSS for coastal zone and water management. The findings of the comparison show, amongst others, that SDSS only provide limited or no support for analysing and evaluating the output generated by SDSS. Also, no or poor support for spatial evaluation is offered which might be one of the reasons why some SDSS are not used. © 2003 Published by Elsevier Ltd.