How much to invest is a fundamental question applying equally well to socially valued public investments as to the private sector. Benefit-cost analysis is the cornerstone of the economic analysis of public policy, and is closely aligned with basic rational choice and market concepts from microeconomics. Information and other constraints often block the direct application of marginal analysis in policy decisions, but the conceptual role of marginalism can still be useful in interpreting cost-benefit analysis. While all policy analysis texts that emphasize the economic dimensions of policy cover the basics of marginal analysis, the sources of market inefficiency, and basic decision rules for policy analysis, the connections between marginal analysis and non-marginal policy decision rules are seldom emphasized. This chapter limits its discussion of marginal analysis to the concepts of optimal quantity and optimal allocation rather than the market based concepts of surplus, equilibrium and elasticity, which are discussed in later chapters. This chapter begins by reviewing marginal and non-marginal concepts and measures for policymaking, and then discusses a set of basic policy decisions that can be informed by these concepts. Student exercises are included and answered in the appendix to the chapter.