Welfare states have increasingly moved from income transfers and passive protection towards service provision, investments in human capital, and proactive interventions. This chapter explores how the social investment framework can be extended with interventions to address its limitations, especially in relation to acute risks and inequalities. We define interventions broadly as strategies or actions that target individuals, groups, or environments to promote well-being, prevent risks, or mitigate existing problems across the life course. Drawing on examples from health, education, and social policy, we illustrate how interventions can be classified by scope, timing, objectives, and target groups. We also discuss methodological approaches to evaluating effectiveness and cost-effectiveness, highlighting both randomised controlled trials and alternative designs. Finally, we examine the challenges of implementation and scaling, noting the long time lags between evidence and practice. Together, these insights demonstrate how interventions, when effectively designed and embedded, can become persistent social investments within modern welfare states.