Contents: 1. Introduction. 2. A primer on quantum statistical mechanics and path integrals. 3. Master equations: a prolegomenon to open quantum systems. 4. Influence functional approach to open quantum systems. 5. Dissipative harmonic oscillator. 6. Dissipative two-state system. 7. Quantum tunneling. 8. Open quantum system at interface with quantum information. 9. Recent trends. References. Index. Current developments in quantum computing have made it essential to consider the role of the ambient environment in the functioning of a quantum system. Consequently, the study of such open quantum systems has gathered momentum in recent years, leading to better understanding of issues such as decoherence and dissipation at the microscopic level. This text provides a window into the current state-of-the art in the study of open, quantum systems, while providing a pedagogic introduction to the basic ideas that makes it accessible to a newcomer to the field. Familiarity with quantum mechanics and statistical physics is assumed. The dissipative harmonic oscillator, as well as dissipation in two-level systems, tunneling, and quantum information are among the topics that are discussed in considerable detail.