Open Quantum Systems : Dynamics of Nonclassical Evolution Chapter 1. Introduction Chapter 2. A Primer on Quantum Statistical Mechanics and Path Integrals Chapter 3. Master Equations: A Prolegomenon to Open Quantum Systems Chapter 4. Influence Functional Approach to Open Quantum Systems Chapter 5. Dissipative Harmonic Oscillator Chapter 6. Dissipative Two State System Chapter 7. Quantum Tunneling Chapter 8. Open Quantum System at Interface with Quantum Information Chapter 9. Recent Trends This book discusses the elementary ideas and tools needed for open quantum systems in a comprehensive manner. The emphasis is given to both the traditional master equation as well as the functional (path) integral approaches. It discusses the basic paradigm of open systems the harmonic oscillator and the two level system in detail. The traditional topics of dissipation and tunneling as well as the modern field of quantum information find a prominent place in the book. Assuming a basic background of quantum and statistical mechanics this book will help readers familiarize with the basic tools of open quantum systems. Open quantum systems is the study of quantum dynamics of the system of interest taking into account the effects of the ambient environment. It is ubiquitous in the sense that any system could be envisaged to be surrounded by its environment which could naturally exert its influence on it. Open quantum systems allows for a systematic understanding of irreversible processes such as decoherence and dissipation of the essence in order to have a correct understanding of realistic quantum dynamics and also for possible implementations. This would be essential for a possible development of quantum technologies