This DFID-funded study on Shock-Responsive Social Protection Systems strengthens the evidence base as to when and how social protection systems can better scale up in response to shocks in low-income countries and fragile and conflict-affected states, thus reducing the need for separate humanitarian responses. The key research question is: 'What are the constraints to social protection systems being more responsive to shocks, and, conversely, what factors would enable social protection systems to become more responsive to shocks?' The study, which ran from 2015-2018, was led by Oxford Policy Management (OPM) in consortium with the Overseas Development Institute (ODI), the Cash Learning Partnership (CaLP) and INASP. This synthesis report consolidates the evidence and lessons learned from the research, drawing on all the case studies and other outputs, including the literature review as well as policy briefs on systems development and monitoring and evaluation. It highlights the key ways in which social protection systems may contribute to mitigate the effect of, or respond to, large-scale shocks, and the opportunities and challenges therein; presents the features of programme design and implementation that determine the efficiency and effectiveness of the response; and summarises lessons on achieving a successful collaboration between the humanitarian, DRM and social protection systems. The individual case study reports are available separately.