The report assesses the potential of developing irrigation in the Niger River Basin under various agricultural scenarios accounting for biophysical and socio-economic variables, and for expected climate change. Irrigation potential is assessed in two parts. The first reviews recent literature in English and French (2010 onwards) on sustainable irrigation potential in the Sahel (i.e. Lake Chad basin, Niger, Senegal Volta River basins). Sahel agriculture possesses a significant irrigation potential. However, estimates fluctuate greatly depending on the scale of irrigation schemes, whether the resource is surface or ground water, expected and actual irrigation costs but also on determinants of success of irrigation schemes, including the varying effects when interacting with other inputs, such as fertilisers. Past, and not always successful, efforts were based on large public irrigation schemes (i.e. river dams and related canals). In a growing number of contexts, investments in small and micro-irrigation systems are identified as more desirable than conventional large schemes. Existing small-scale irrigation systems in the region are known to be developing however limited systematised evidence exists. The realisation of this potential is very sensitive to the costs of irrigation, among the highest in the world, with some technologies more sensitive than others (i.e. small river diversions). Moreover, irrigation potential is influenced by synergies among irrigation and other agricultural production technologies – it is maybe worthwhile to recall that irrigation potential is not a static concept, but it is contingent on levels of other inputs. Hence, irrigation investments need to be put in the broader context of productivity enhancement, rural development efforts and global changes such as urbanisation The development of irrigation in the Sahel and in the Niger River basin in particular is a key intervention area for agriculture and development policy in general. Current policy identifies irrigation development as an instrument fostering food security. However, from the angle of optimization, rainfed agriculture retains the larger potential for development when looking at costs and overall potential profits. Moreover, support to the development of irrigated agriculture needs to be fully integrated with a relevant and adapted support to agriculture in general, particularly with regards to how it mitigates risk. Access to irrigation is expected to expand farmers' production opportunities. It mitigates production risks, even in low quantities as crop-saving irrigation. By reducing risk, it encourages farmers to make more intensive use of inputs and land. Moreover, this dynamic effect is also influenced by the type of irrigation systems accessed. For example, the literature has identified that farmers which have some off-farm income are particularly interested in investing in agriculture if irrigation is made available, whereas other groups may be interested in improving first their access to credit for farm inputs with then a view on irrigation. How production risks are perceived need to be clearly identified so that the irrigation systems fostered can be seen as risk-reducing Functioning supply chains would also make irrigation more profitable as they reduce losses of potentially more valuable products from irrigated agriculture and enhance market access. Recently, registered regional increases in groundwater storage have been associated to diffuse recharge, partially compensating for groundwater withdrawal associated with irrigation development. Hence, hinting at some level of sustainability in the use of groundwater for small-scale irrigation in the Sahel, despite the risks associated with salinization. The second part focuses on the Niger Basin to assess and quantify its irrigation potential through modelling. The model uses static biophysical and socio-economic indicators in model optimising profits of mainly small holder farms under 4 possible agricultural scenarios with distinctive productivity levels. In general, the projected irrigated area does not evolve much between scenarios mainly because of high productions costs associated with increased irrigation. Although irrigation potential is theoretically large, investing in both irrigated and rainfed input intensification offers the largest potential gains. The results for total irrigation potential in terms of farmed area are in the range of 0.6-09M hectares, from the estimated current 0.53M hectares of irrigated land under the most productive scenario in terms of agricultural yields. However, even the most yielding scenario results of the current study are significantly lower than previous estimates developed in the literature, and depend on assumed irrigation and input costs. The specific strengths of this new estimation are that of using input costs from recent agricultural surveys (i.e. LSMS-ISA) along with crop suitability maps. Its main limitation is that is does not distinguish between irrigation technologies and related costs, constraining estimates to a generic (gravity) irrigation. In turn, the expansion of agriculture is exogenously determined and does not depend upon the variables analysed.