The effectiveness of Environmental Assessment (EA) has been intensively discussed in academia and practice. However, few studies provided a longitudinal analysis of a country or examined in-depth the interactions with the institutional context. This article aims to contribute to the understanding of EA-effectiveness by examining the interaction of EA with planning and decision-making institutions. Therefore, we present an in-depth historical analysis of how EA together with the planning and decision-making system evolved over time, using Estonia as a case. We developed an analytical framework to analyse the various effectiveness dimensions of EA (procedural, substantive, transactive, legitimacy, and knowledge& learning) and their interactions with the broader institutional setting of planning and decision-making, and conducted document analysis (of regulations, policies and evaluations), interviews and a focus group, reviewing the period between 1988 and 2024. Important findings include that the dominance of the effectiveness dimensions is dynamic characterized by the interaction between the EA-system and the broader institutional setting of the planning system. Furthermore, there is interaction between the different dimensions of effectiveness over time. This means that EA effectiveness cannot be fully understood by examining one single effectiveness dimension nor by considering EA in isolation; the broader institutional context must be considered. To improve EA effectiveness, it is crucial to acknowledge this, and to address multiple effectiveness dimensions as well as the broader institutional setting. Perhaps the key to enhancing EA effectiveness lies beyond EA itself, which aligns with its original role as an instrument to aid decision-making and planning.