A quantum system in contact with its environment is called an open environment system. The aim of this book is to present certain aspects of contemporary theory on open quantum systems. It presents numerous updated topics, like modeling of quantum noise, detecting quantum entanglement, quantum communication procedures, computational intricacy in the evaluation of quantum operations etc., along with discussions on light propagation in optically dressed media, entropy and information measures for quantized electromagnetic fields media. The book aims to serve as a useful source of information for students as well as researchers interested in the extensive fields of open systems and quantum optics.