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Prevention, and the act of preventing, refer to proactive actions aimed at reducing the likelihood, severity, or duration of adverse events. The goal is to intervene before problems arise or worsen, rather than treating them after they occur. The concept encompasses a broad range of activities across health, safety, environment, crime, and social policy.

Public health commonly organizes prevention into three levels: primary prevention aims to stop problems before they start (for example, vaccination or health education to reduce risk factors); secondary prevention focuses on early detection and intervention to halt progression (such as screening programs and early treatment); tertiary prevention seeks to lessen damage and disability once a condition has developed (for instance, rehabilitation and management to prevent complications). Similar categorizations exist in other fields, including safety engineering, criminal justice, and disaster risk management.

Effective prevention relies on evidence, risk assessment, and coordinated strategies. Methods include policy and regulation, population-wide programs, preventive medicine, environmental and engineering controls, and community engagement. Economic considerations, equity, and ethical concerns often influence design and implementation, as resources are finite and benefits can vary across groups. Measurement typically uses indicators such as incidence and prevalence reductions, delayed onset, or decreased severity.

Historically, prevention has evolved from individual-level interventions to comprehensive, multi-sector approaches. Contemporary prevention science emphasizes intervention effectiveness, scalability, and sustainability, with emphasis on reducing disparities and building resilience. Related concepts include health promotion, risk reduction, and preventive care.