  Perturbative quantum field theory usually uses second quantisation and
Feynman diagrams. The worldline formalism provides an alternative approach
based on first quantised particle path integrals, similar in spirit to string
perturbation theory. Here we review the history, main features and present
applications of the formalism. Our emphasis is on recent developments such as
the path integral representation of open fermion lines, the description of
colour using auxiliary worldline fields, incorporation of higher spin, and
extension of the formalism to non-commutative space.
