The 2004 EU enlargement and related Europeanisation processes supported the development of stagnated interest group systems in many ways, including with respect to the professionalisation of mainly voluntary-based organisations in Central and Eastern Europe. In the pre-membership period and initial years after joining the EU, national interest groups from Central and Eastern Europe chiefly relied on EU-level interest groups for important information, knowledge, and know-how concerning EU policymaking, whereas 20 years of membership has today established them as equal partners and co-decision-makers. The article elaborates on the Europeanisation of interest groups in the Central and Eastern Europe region from the start of the process of accession to the EU, with three case studies in focus: Lithuania, Poland, and Slovenia. The main research question is: In which different ways has the Europeanisation process influenced interest groups in the region? To address it, the article builds on Johansson and Jacobsson’s (2016) typology of the Europeanisation of interest groups. Six exploratory factors were examined in this regard: (a) contacts with EU policymakers and institutions, (b) interest in EU policymaking, (c) funding received from EU projects and programmes, (d) networking with EU umbrella organisations, (e) participation in open consultations, and (f) the relationship of the group with members. To study the effects of Europeanisation processes in selected countries, web survey data gathered from national interest groups as part of the Comparative Interest Groups Survey project were used. Our results show that interest groups from Central and Eastern Europe have become “European” in a range of ways. Regulatory and discursive Europeanisation is most typical for Polish interest groups, identity Europeanisation for Lithuanian interest groups, and financial and participatory Europeanisation for Lithuanian and Polish interest groups, while organisational Europeanisation has the strongest effect on interest groups in Slovenia.