Quantum Networks is focused on density matrix theory cast into a product operator representation, particularly adapted to describing networks of finite state subsystems. This approach is important for understanding non-classical aspects such as single subsystem and multi-subsystem entanglement. An intuitive picture evolves of how these features are generated and destroyed by interactions with the environment. This second edition has been revised and enlarged. For better clarity the text has been partly reorganized and figures and formulae are presented in a more attractive way.