Technical Abstract: Crop yield varies within a field because conventional farming manages fields uniformly with no consideration for spatial variability. Site-specific management units (SSMUs), a key component of precision agriculture, have been proposed as a means of handling the spatial variability of various factors (i.e., soil, climate, management, pests, etc.) influencing crop yield variation in order to increase crop productivity. It is the objective of this magazine article to provide an overview of the work by Corwin and colleagues (Corwin and Lesch, 2003, 2005b; Corwin et al., 2003) that lead to the delineation of SSMUs. Equipment, methods, and techniques are presented for delineating SSMUs using geo-referenced apparent soil electrical conductivity (ECa) measurements. The approach uses the ECa measurements to locate where soil samples are to be taken to characterize the variability of soil properties influencing the variation of cotton yield within a 32.4-ha field. A crop yield response model is developed and maps of SSMUs are prepared using a geographic information system of the spatial soil and crop yield information. The methodology for delineating SSMUs can be used whenever ECa correlates with yield. The ability to create maps of SSMUs provides the vital information for variable-rate technology (i.e., site-specific irrigation water application).