The book Ṛtu Vidyā emerged in search of answers to questions asked by adolescent girls and women in India during the author's interactions with them as part of Menstrual Health workshops, conducted over a span of a decade across rural India. In an attempt to decode menstrual practices, the author undertook a journey across India and studied various indigenous knowledge systems such as Ṣaḍ-Darśana, Āyurved, Tantra, Cakra, Yōg, Āgama Śāstra, Jyotiṣa Śāstra, and several sub-texts from these categories. As a result, the book goes beyond just describing cultural practices and takes a deep dive into explaining the scientific and logical reasoning behind the origin of these practices. This book is for all Indian women who have unanswered questions pertaining to menstrual practices, for menstrual researchers who will find a treasure trove of potential areas for research pertaining to menstrual health, for sportswomen to discover the ancient techniques that worked in sync with women's periods and not against it, and also for the feminist who assumes that cultural practices around menstruation are a taboo that needs to be done away with.The correct understanding of the science behind menstrual practices, as given in this book, will help women prevent menstrual difficulties, develop a positive attitude toward menstruation, and learn to work in sync with nature's cycles.Ṛtu (pronounced as ruthu) is one of the terms for menstruation in Sanskrit. Vidyā means knowledge. Ṛtu Vidyā is the author's attempt to bring together various indigenous knowledge systems that provide information about the science of menstruation, which is relevant even to this day.