While seed security is key to food security, concrete means for building resilient seed systems remain unexplored in research and practice. A new toolkit, the Seed System Security Assessment (SSSA), examines what actually happens to seed systems during crises and highlights specific features that foster or undermine resilience. Drawing evidence from SSSAs in contexts of political and civil conflict (Zimbabwe and South Sudan), earthquake (Haiti) and drought (Kenya), the article shows that seed systems prove to be relatively resilient, at least in terms of meeting farmers' planting needs for the upcoming season. Altering crop profiles, making use of multiple delivery channels, and innovating (for example, with new barter mechanisms) all become key, as does mobilizing cross-scale seed supply linkages. However, despite short-term survival, in the medium term, both formal and informal seed systems will have to be transformed to address agro-ecological and farming system challenges, partially shaped by global environmental changes. Key is that formal seed systems will play a catalytic but supporting role, with the onus on resilience response lying within informal systems, and especially with local markets and their traders. Also key is that achieving seed security in fluctuating environments will hinge on developing resilience-linked information systems which put as much weight on helping farmers strategize as on delivering the planting material itself. The article defines seed system resilience, identifies eight principles linked to processes that build such resilience, and makes 15 practical recommendations for enhancing seed system resilience in the short and medium term. Finally, drawing insights from seed systems, processes central for building resilience in other development sectors are highlighted.