  Localization properties of scalar single particle states are analyzed by
explicit calculational examples with a focus on the massless case. Problems
arising from the non-existence of relativistic particle position operators
respecting the causal structure of Minkowski spacetime are illustrated by
exploring the conflicts arising from localization and causal properties
commonly imposed on single particle states. These topics necessitate the
introduction of quantum field theoretical localization concepts and are
scarcely discussed and often misinterpreted in the literature.
