Developments in entanglement theory and applications to relevant physical systems

This Thesis is devoted to the analysis of entanglement in relevant physical systems. Entanglement is the conducting theme of this research, though I do not dedicate to a single topic, but consider a wide scope of physical situations. I have followed mainly three lines of research for this Thesis, with a series of different works each, which are, Entanglement and Relativistic Quantum Theory, Continuous-variable entanglement, and Multipartite entanglement.