Field theory is the ultimate consequence of the attempts to reconcile the principles of relativistic invariance with those of quantum mechanics. It is not too difficult, with a lot of hindsight, to understand why a field needs to be introduced. This is not an attempt to do justice to history-and perhaps one should spare the student the long struggle to arrive at a consistent formulation, which most likely has not completely crystalised yet either-but the traditional approach of introducing the concept is not very inspiring and most often lacks physical motivation. In the following discussion I was inspired by Relativistic Quantum Theory from V.B. Berestetskii, E.M. Lifshitz, and L.P. Pitaevskii (Pergamon Press, Oxford, 1971). The argument goes back to L.D. Landau and R.E. Peierls (1930).