An inventory was made of the alien macroinvertebrates occurring in Flanders. To this end large collections of biological samples were investigated and supplemented with own sampling campaigns. Mainly fresh and brackish surface waters were investigated. Three new macroinvertebrate species for Flanders were discovered. In total 41 alien macroinvertebrates were encountered in fresh and slightly brackish surface waters in Flanders. Additionally, 24 alien macroinvertebrate species have been reported for the Belgian part of the North Sea and its adjacent estuaries. Most alien macroinvertebrates belonged to the crustaceans and molluscs. Over 2,500 samples containing macrocrustaceans were identified to species level, which allowed us to accurately map their distribution in Flanders. Alien species found in the fresh and brackish water environment, mainly originated from the Ponto-Caspian area and North-America followed by Asia and South- and East-Europe. This overview shows that alien macroinvertebrates are ...