On the macroscopic limit of quantum systems

This thesis is devoted to the analysis of composite bipartite systems where at least one of the components is macroscopic, so that it can be in the border region between quantum and classical behaviours. Combining the tools of the Open Quantum Systems theory with approaches from quantum field theory, formally describing the conditions for a classical theory to emerge as the number of the elementary quantum components of the system described by a quantum theory increases, we deal with the fundamental issue of the quantum-to-classical crossover and we investigate the general idea that a classical environment is a macroscopic quantum environment.