Public health policies sometimes make demands on individuals who do not stand to benefit from the policies, and they sometimes interfere with liberty even when they do benefit the individuals in question. In such instances, a moral justification for a public health intervention is required. This chapter sets forth five justifications for public health interventions: (1) overall benefit, (2) collective action and efficiency, (3) fairness in the distribution of burdens, (4) prevention of harm (the harm principle), and (5) paternalism. The chapter discusses each justification in turn, posits that often more than one justification applies to a given policy, and argues against frameworks that place disproportionate attention on conflicts between liberty and health.