The extent to which Environmental Assessment (EA) contributes to incorporating environmental values and objectives into decision-making (i.e. the effectiveness of EA) has been subject to much research. Still relatively little is known about how the effectiveness of EA is influenced by the specific features of EA systems and their context. International comparative research can shed more light on these relationships. In this paper we report on a survey of EA in Flanders, taking a similar approach as previous surveys in the Netherlands, UK and Denmark. We observe that the effectiveness of the Flemish project-based EA (EIA) is comparable to that in the other countries, whereas the Flemish plan-based EA (SEA) is more influential than the Dutch one (no data on Denmark and UK). As in the other countries, EA in Flanders has an influence on decision-making both before and after the EIA has been completed. According to respondents to the surveys, in all four countries the legal requirement is the main explanatory factor for EA effectiveness. The mechanisms by which EA characteristics and other factors contribute to EA effectiveness seem rather country-specific, however. Rather than trying to isolate the individual influence of factors we encourage more in-depth, qualitative and case-study based follow-up research in order to better understand the complex interplay between factors related to the EA system itself, how it is applied in practice and influenceds from its specific context.