The dissertation aims at understanding and explaining the existence of variation in sustainable compliance with EU legislation in two similarly rule-taking countries. The cases under examination are Hungary and Poland which have experienced a similar historical background, similar environmental problems and have been subject to similar EU conditions and requirements for accession. Nevertheless, the EU Annual Progress Reports and the Tri-Annual Monitoring Reports showed a variation in their compliance with European environmental requirements. The existing literature has explained this divergence by taking a supply-side approach, focusing on those state actors and incumbents who could decide to supply compliance or not. In particular, researchers of compliance and of Europeanisation have focused on differences in capacity limitations or incentives to domestic actors. These supply-side approaches, however, do not seem to fully explain the existing divergence between the performances of Hungary and Poland nor do they sufficiently tackle the issue of "sustainable compliance" in the post-Accession period. In my analysis, I instead explain variation in sustainable compliance by exploring demand-side explanations. To this end, the thesis explores the hypothesis of demand for compliance emerging on the part of stakeholders who recognise its potential for profitability and, thus, influence its sustainability. Its starting point is the Tsebelis' study on stakeholders which describes them solely as "veto players" along the road to compliance; however, this analysis demonstrates that there is also another dimension to the influence they may have. I build my hypothesis around the existence of such factors as market incentives and pre-existing cooperative strategies that make compliance convenient for stakeholders. Moreover, I consider the role played by external assistance and the existence of alliances between external and domestic stakeholders to improve the overall compliance performance of less-regulated countries. The study proves the significance of market incentives and pre-existing cooperative strategies in fostering sustainable compliance while showing how the two strong explanatory variables are interlinked: compliance is not a "business" per se. It has a potential to be made a "good deal" via cooperative strategies among diverse stakeholders creating a win-win settlement.