Adoption of non-labor agricultural inputs remains low among small-scale farmers in many low-income countries. Accurate measurement of the quality of these inputs and quantities used is essential for assessing economic returns, understanding the drivers of agricultural productivity, and proposing and evaluating policies for increasing agricultural production. We review evidence regarding the quality of planting material, fertilizer and pesticides used by small farmers in low-income countries with a focus on Sub-Saharan Africa where the literature is most extensive. We distill four key findings. First, empirical evidence to date has centered on a limited set of agricultural inputs and locations. Second, some of this evidence is difficult to evaluate or may be misleading because best testing practices either were not followed or were not sufficiently documented. Third, while farmers are generally suspicious about input quality and therefore may hesitate to invest, these beliefs may exaggerate the severity of the problem. Farmers may attribute too much blame to poor quality inputs for bad crop yield outcomes. Fourth, most evidence comes from on-farm or in-shop samples; where input quality issues emerge at these downstream stages it is typically unclear where and how problems enter the upstream supply chain. We argue that while accurate documentation of measured and perceived agricultural input quality is important, the marginal productivity effects of input use hinge on the timing and method of application and on a host of complementary inputs (e.g., soils, moisture, labor). We conclude with specific priorities for future research that are linked to these key findings.