Agricultural productivity in Sub-Saharan Africa (SSA) remains low and is falling farther behind other regions of the world. Although agricultural output growth in the region has accelerated since the 1990s, this has been primarily due to resource expansion rather than to higher productivity. Yet there is evidence that agricultural productivity growth has improved in some countries. Enhanced productivity is correlated with investments in agricultural research, wider adoption of new technologies, and policy reforms that have strengthened economic incentives to farmers. Many of the technological improvements have come from the Consultative Group for International Agricultural Research (CGIAR) centers. Benefits from the CGIAR in SSA are estimated to be over $6 for each $1 invested. Returns to national agricultural research are also robust, at least for large countries. But overall investment in agricultural research has remained low, and increases in research capacity will likely be necessary to significantly accelerate agricultural growth in the region. Other constraints to agricultural productivity include government policies that reduce earnings in the farm sector, the spread of the HIV/AIDS virus, and armed conflict within and between countries.