This chapter outlines an empirical approach to understanding the determinants of farmers’ access to, and use of, crop genetic resources (CGR) and the impacts of farmer behavior on crop populations. These are critical to modeling the current status of agricultural biodiversity on farm, as well as in designing and implementing effective programs for the in situ conservation of agricultural biodiversity. A case study from applied fieldwork in Mexico is also provided to illustrate the concepts presented. Empirical evidence is presented to demonstrate the type of data to be collected from farm level surveys that is necessary to measure population level processes from farmer behavior.