Synopsis In the first three volumes on "The Treatise on Indian Medicinal Plants" classified informations have been given on 352 plant species used in the traditional system of medicine. This volume describes 127 plants covering twenty one botanical families maintaining the same pattern of presentation as in previous volumes. In order that the readers may be conversant with the Ayurvedic terminologies, the chapter on the "Basic Concepts of Ayurveda" has been retained also in the present volume. Index of plant species of volumes 1-3 have also been incorporated. Several plants included in this volume are well-known sources of herbal drugs and drug intermediates. A few deserve to be highlighted.