The complex and reinforcing relationship between fragility/conflict and economic development is widely recognised, though not entirely understood. While trend observations suggest that low economic growth increases the risk of conflict and conflict reduces economic growth, this is not the case everywhere. Even where it does hold, the characteristics of economic growth and conflict differ widely across contexts. Collier et al.’s (2009) opportunity-grievance-feasibility framework is generally considered useful for conceptualising the relationship between economic development and conflict, but opinions differ onwhich driving factors are most important and how they interact. The underlying causal mechanisms are disputed, as are the definitions of concepts (e.g. fragile and conflict-affected states) and the methods used to measure them (e.g. growth in GDP). It is clear, however, that economic development and conflict interact at all levels throughout all stages of conflict and fragility in ways that have substantial, but diverse, effects on individuals, households, and private and public institutions. This guide explores theory and evidence of these interactions in various contexts. Economic interventions in FCAS seek to address economic issues while contributing to wider statebuilding and peacebuilding objectives. Yet programme design is often based on assumptions that are not universally accepted or understood.