This chapter conceptualizes computer simulation and policy-making at the science-policy interface exploring boundaries between scientific knowledge production and political action orientation. The conceptualization entails four layers. First, compatibilities of scientific simulations with the policy-making system rely on key characteristics of modelling meeting policy's reasoning, forward-looking and decision oriented needs. Simulations meets these needs with their capability to reduce complexity, compare options, analyse intervention effects, deliver results in numbers, and carry out trial without error. Second, from a systemic perspective, simulations serve as a knowledge instrument contributing to secure and uncertain knowledge and the known unknowns. Simulations also enable, amplify and feedback communication. Third, taking an impact perspective, the policy use of simulations differentiates in instrumental, conceptual, strategic and procedural use patterns. Finally, evaluation and assessment of simulations by decision-makers follows several simulation-inherent and simulation-contextual criteria.