Economists have long studied the diffusion of improved agricultural technologies, often aiming to understand and relax the constraints that discourage their adoption among smallholder farmers. While this effort has documented and explored a long list of on-farm market constraints, the role of agrodealers in agricultural input markets has received far less attention—a critical blind spot. We review the empirical literature on input markets in low-income countries with a focus on Sub-Saharan Africa. We argue that the relative sparsity of this literature reflects limitations in our workhorse models of the household and the firm combined with a supply-side data gap due to infrequent systematic surveys of agrodealer firms. Consequently, we understand too little about the diverse input supply chains that culminate with agrodealers—large and small—marketing key inputs to farmers. We synthesize current findings and articulate a research agenda centered on agricultural input markets, including implications for research methods.