Competition in plants occurs in two physically separated spheres -- belowground and aboveground. Aboveground, plants primarily compete for light, but belowground competition may be much more complicated, potentially involving water, space, and any of more than 20 mineral nutrients required for plant growth. Not only are soil resources diverse but they are temporally and spatially variable, and little is known about the consequences of this heterogeneity for population or community structure. Here we consider how spatial nutrient heterogeneity might affect the outcome of plant competition. We first review spatially explicit models of plant competition and give particular attention to how resource heterogeneity has been incorporated into them. We then report experiments in which we examined the consequences of nutrient heterogeneity for both intraspecific and interspecific competition. By obtaining measures of lateral root spread in populations of annual plants, we have learned a great deal about the structure of the populations belowground and how that structure changes with the spatial distribution of nutrients. We relate our results to assumptions made in the construction of spatially explicit competition models.