Abstract: Weeds are a perpetual challenge in organic agriculture. The international PRODIVA project studied the effect of crop diversity on the diversity of weed communities, hypothesizing that by increasing the weed diversity, the development of problematic weeds will be mitigated. In order to target the right “problematic” weed species, a preparatory study was conducted to list the most problematic weed species in spring sown cereals in the countries involved in PRODIVA. Therefore, a literature review was conducted in all participating countries, collecting local sources including grey literature, and the opinion of local extension services and other weed experts. From this a list of the most problematic weeds was selected for each country. To explore how far the expert data base reflected the actual situation in the field, these preparatory lists were compared to the actual weed species occurrence found during the field study. Although the expert data often reflected the reality, some species were under- and overestimated, especially non-competitive and monocotyledonous species, respectively. Here adjustment of preconceived notions would be appropriate.