The Third Edition of this popular textbook is based on the latest syllabus of the Ability Enhancement Compulsory Course on Environmental Studies, prescribed by the University Grants Commission (UGC) under the new Choice-based Credit System. It also includes additional topics included in the syllabi of some universities. The book is meant for undergraduates of all disciplines. Assuming very little background in mathematics or science, it gives a balanced presentation of the major issues and concerns related to the environment. This book will also be useful for those taking the UPSC Civil Services Examinations, since it covers the ecology/environment topics of the Preliminary and Main Examinations.