This chapter reviews the photosynthetic response of plants to drought stress. It first describes the indicators for photosynthetic traits, then goes on to discuss the different factors that can affect photosynthetic efficiency. The chapter also considers how drought stress can affect photosynthetic assimilation efficiency and examines the photosynthetic metabolic pathways of photosynthesis associated with drought tolerence in crops. The chapter also looks at how maintaining photosynthetic rate under drought stress can contribute to crop drought tolerance. A section on how the engineering of C4 traits into C3 plants could improve crop drought tolerance is also provided, followed by an overview of drought tolerance genes related to photosynthesis. The chapter examines the use of quantitative trait loci (QTL) mapping for photosynthetic traits, considers how the stay-green phenotype can lead to increased crop yield under drought conditions and reviews how introducing this stay-green trait can enhance photosynthesis under drought stress. A case study on the enhancement of sorghum tolerance to post-flowering drought stress using stay-green QTL is also included.