This article aims to explain Europeanization processes among advocacy organizations from four post-communist states—Poland, Hungary, Slovenia, and the Czech Republic—which have been subject to strong Europeanization pressures before and after the European Union accession. The authors aim to identify how Central and Eastern European organized interests have adapted their organizational logics to a changing environment in the post-enlargement phase. Specifically, we address the following questions: How do various levels and dimensions of Europeanization of interest organizations differ across policy fields and countries and what determines this diversity? What are the strongest predictors of the specific forms of Europeanization? Against this background, we test how the specific characteristics of CEE advocacy organizations correlate with the different dimensions of Europeanization—its intensity, ways of occurring, variants, determinants, and outcomes.