The concept of a zone of influence, the area over which a plant alters the environment, forms the basis of many models of plant competition. Because of logistical difficulties, we actually know little about the sizes and shapes of zones of influence belowground. Here we advocate obtaining data on plants' belowground zones of influence including the length and distribution of lateral roots, in order to understand better how plants respond to their abiotic soil environment and to other plants. We provide several examples from recent work. First, we present an analysis of a large global data set which shows that maximum lateral root spread correlates with canopy size but that for a given canopy size, maximum lateral root spread is greater in arid environments and in coarse textured soils. Second, we use an experiment with the weedy annual Abutilon theophrasti to show how using nutrient analogs as tracers yields information about lateral root distributions within populations. In our experimental populations the belowground zone of influence extended well beyond the closest neighboring plants. Overlap in zones of influence increased in nutrient patches. Third, we propose a new conceptual model of belowground zones of influence based on these and other data sets. The model assumes that the probability of resource uptake or competing with a particular neighbor declines with distance from the stem but that considerable uptake at great distances from the stem is still possible. It also allows for plasticity in root distributions as might occur in spatially heterogeneous soils. Finally, we suggest ways in which better information on the shapes and sizes of belowground zones of influence should prove more generally useful.