Effective social protection is increasingly as essential to supporting affected populations in situations of protracted instability and displacement. Despite the growing use of social protection in these settings, there is comparatively little rigorous research on what works, for whom, and why. This special issue contributes by adding seven high-quality studies that raise substantially our understanding of the role of social protection in fragile contexts and in settings of forced displacement and migration. Together, these studies fill knowledge gaps, help support informed decision-making by policy-makers and practitioners, and demonstrate that impact evaluation and the analysis of social protection in challenging humanitarian settings are possible. The studies provide evidence that design choices in implementation, such as which population to target, choice of transfer modality or which messages are delivered with programmes, can make a substantial difference in the realisation of positive benefits among vulnerable populations. Furthermore, the findings of the studies underline the relevance of tailoring programme components to populations, which may benefit more or less from traditional programme implementation models. Humanitarian challenges of protracted fragility and conflict-related crises and the more recent unprecedented migration and refugee movements around the globe underscore the need to break down the barriers between humanitarian and development work. Ongoing and new crises left an estimated 206.4 million people in 81 countries in need of international humanitarian assistance in 2018 (Development Initiatives, 2019). Over 65 million individuals were estimated to have been forcibly displaced worldwide in 2015 as a result of prosecution, conflict, generalised violence, or other human rights violations (United Nations High Commissioner for Refugees [UNHCR], 2016). In fragile contexts and protracted crises, such as in Afghanistan and Somalia, responsive long-term systems are needed to reach affected vulnerable populations consistently. Acute and extended crises, such as in Syria, have contributed to migration flows, which also highlight the need for long-term solutions in countries of destination. Social protection is increasingly considered as a policy response in contexts of fragility and displacement. In nonfragile contexts, extensive evidence demonstrates that social protection helps reduce poverty and inequality, enhances livelihoods, and has long-term positive impacts on human capital development (