Chronobiology is an emerging branch of life sciences. It deals with the study of biological rhythms. All biological events necessarily takes place in the dimension of time. The capacity to undergo rhythmic oscillations is a characteristic of all living organisms. The books serves as the introduction to the subject. It offers necessary fundamentals and frameworks to get further with basic or applied research in chronobiology. Since the Nobel Prize awarded in 2017 for understanding of molecular mechanisms of circadian rhythms, both society and scientific community started taking the subject seriously which is evident by sharp rise in number of publications involving temporal understanding of various body systems. Understanding mechanism of biological clock is much more relevant in modern 24x7 lifestyle which leads to circadian disruption. We know how to handle pathogen borne diseases but lifestyle related diseases cannot be treated successfully without chronobiological understanding of the body mechanisms which is currently lacking in our diagnosis or treatment. This book has three sections. First two sections provides the technical information of the subject which may require basic background of school level biology to understand the terminology. Third section is more about the application of chronobiology in human welfare and does not have much technical terms. All the chapters are provided with references, glossary and exercises to understand the subject better. We encourage readers to get engaged with the subject with the help of exercises and also read the additional information available in references.