Based on research on the effectiveness of Strategic Environmental Assessment (SEA) and the critical factors determining it, numerous questions regarding SEA and its effectiveness have come to light. In this paper, we address a range of these questions based on a review of 39 peer-reviewed scientific articles. Questions are examined such as how effectiveness is defined and how the critical factors affecting it are determined. The concept of effectiveness is broadly accepted, but its meaning differs between different schools of thought. Effectiveness is clearly kept apart from the concept of quality, but in many definitions of SEA effectiveness, we have seen the concepts presented as overlapping and developing through time. At any point in time, a multitude of meanings of effectiveness thus exist, sustaining the idea of Thomas Kuhn that this kind of social science is not really cumulative. To overcome these limitations, we present a view of how these different concepts can be presented in a number of logical and historical steps. Based on the critical factors and their concentration into broader concepts, it targets to the formulation of a general, more comprehensive model that can describe the interrelationship between general- and stage-specific critical factors. It is concluded that “comprehensive models of causation” exist which condense a broader range of critical factors into only 4–10 main topics that describe the most pivotal forces at play. It may be possible to facilitate a common understanding of the nature of SEA and thus invite future research.