Although root system studies are generally conceded to be important, the difficulties in such study are sufficiently daunting that root systems have received comparatively little attention in physiological ecology. The problems involve, in part, the sizable labor and time investment, the variability in root locations and activity, and the inadequacies of many root measures. The difficulties are often exacerbated for very fine roots and deep root systems. In spite of these difficultties, a judicious selection of experimental approaches, including indirect assessments, can yield meaningful results and can contribute significantly to an understanding of plant function in the field. Many new techniques have become available in the last few years. This chapter outlines a broad range of techniques for assessing root system structure and function in the field. Boehm (1979) has thoroughly described the classical, and still useful, root evaluation techniques. Emphasis here is, therefore, directed to newer techniques not covered to a great extent by Boehm (1979). This chapter treats methods as they relate primarily to the study of root systems in the field. Individuals working with root systems have displayed a certain proclivity to coin new terms for the ecological lexicon. In this chapter one encounters terms such as a 'hydropneumatic elutriation system', a 'microrhizotron' and a 'perforon'. Of course, we feel a certain obligation to define such terms but at the same time to employ more conventional English descriptors.