  Constructing observables that describe the localization of relativistic
particles is an important foundational problem in relativistic quantum field
theory (QFT). The description of localization in terms of single-time
observables leads to conflict with the requirement of causality. In this paper,
we describe particle localization in terms of time-of-arrival observables,
defined in terms of the interaction between a quantum field and a measuring
apparatus. The resulting probabilities are linear functionals of QFT
correlation functions. Specializing to the case of a scalar field, we identify
several time-of-arrival observables differing on the way that the apparatus
localizes particle-detection records. Maximum localization is obtained for a
unique observable that is related to the Newton-Wigner position operator.
Finally, we define a measure of localizability for relativistic particles in
terms of a novel time-energy uncertainty relation for the variance of the time
of arrival.
