Quantum Field Theory (QFT) is the general framework for the description of the physics of relativistic quantum systems, notably of elementary particles. It is the synthesis of Quantum Theory and Special Relativity, supplemented by the principle of Locality in space and time, and by the Spectral Condition in energy and momentum. Algebraic QFT (AQFT) emphasizes the role of algebraic relations among observables which determine, rather than quantum fields proper, a physical system.