In this book, we have studied the behaviour of nanoscale quantum systems interacting with a macroscopic bosonic environment. Motivated by considerations from condensed matter physics and biological systems, we highlighted the need to go beyond the usual Markovian assumption and, furthermore, beyond the approximation (often taken for granted) of local independent baths. Indeed, miniaturised nanodevices have several working units in a small volume that thus dissipate energy in the same region of space.