The potential importance of deep root systems to plant function has long been recognized. Until recently there have been few attempts to actually quantify the importance of deep roots in securing water and nutrients. The development of new methods and approaches which allow direct and indirect study of deep root function offers new and exciting possibilities. Modern drilling equipment can extract deep soil samples with minimal disturbance to roots and the soil biota. The application of stable isotope techniques using variations in natural abundance is especially exciting since analysis of easily collected aboveground tissues provides information on the function of deep root systems. Information on symbiotic nitrogen fixation and the identification of water sources used by the plant can be obtained with this approach. Although poorly described in natural systems, the structure, species composition and population densities of soil rhizosphere biota may proved information on the relative extent of root activity at a particular soil depth. The application of more than one of these approaches to a single system provides the best opportunity to better understand the functional importance of deep roots to plants.