  The pseudoscalar particles pions, kaons and the $\eta$-particle are
considerably lighter than the other hadrons such as protons or neutrons. Their
lightness was understood as a consequence of approximate chiral symmetry
breaking. This led to current algebra, a way to express the relations imposed
by the symmetry breaking. It was realized by Weinberg that because of their low
mass, it is possible to formulate a purely pionic (effective) field theory at
experimental energies, which carries all information on the (non-perturbative)
dynamics, symmetries, and their spontaneous breaking of quantum chromodynamics
(QCD) and allows for systematic calculations of observables. In this review, we
trace these developments and present recent activities in this field. We make
the connection to other effective theories, more generally introduced by
Wilson, as approximate field theories at low energies. Indeed, principles and
paradigms introduced first for pions have become ubiquitous in particle physics
and the standard model. Lastly, we turn to the latest development where the
present (fundamental) standard model itself is considered as an effective field
theory of a - yet to be formulated - even more fundamental theory. We also
discuss important techniques that were developed in order to turn chiral
perturbation theory into a predictive framework and briefly review some
connections between lattice QCD and chiral perturbation theory (ChPT).
