The study examines the history of strategic decision-making concerning water protection in Helsinki, 1850–2000. We identified five major strategic decisions that occurred during the study period. The results indicate that strategic decision-making evolves in long-term policy cycles that last on average 20–30 years. New policy cycles are caused by paradigm shifts. Paradigms are shared and predominant ways of understanding reality that help when groups must act to solve common and complex environmental problems. However the internal structure and external dynamics of paradigms are contradictory. Although paradigms serve initially as means to redefine problems and find creative solutions, as time goes by each paradigm seems to become also a barrier that restricts the introduction of new ways of thinking and acting. The power of paradigms lies in the fact that they can be defined as scientific but also social, political, or cultural agreements depending on the context.