  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.
