Tibetan traditional medicine is a complex and heterogeneous system of healing, based upon the blending of several traditions from prominent Asian medical sciences. The authors of this article conducted fieldwork over a period of 16 months, analyzing how medicinal substances are identified and used by Tibetan medical practitioners. They found that these practitioners typically distinguish medicinal plants through such factors as plants' minute morphological features, plants' taste and scent, and environmental indicators. The authors further explain the inter-related influences of Tibetan medical texts, doctors' individual knowledge and traditions, and common categories of Tibetan materia medica in the classification and use of medicinal plants.