  Almost 50 years after the discovery of gluons & quarks, we are only just
beginning to understand how QCD builds the basic bricks for nuclei: neutrons,
protons, and the pions that bind them. QCD is characterised by two emergent
phenomena: confinement & dynamical chiral symmetry breaking (DCSB). They are
expressed with great force in the character of the pion. In turn, pion
properties suggest that confinement & DCSB are closely connected. As both a
Nambu-Goldstone boson and a quark-antiquark bound-state, the pion is unique in
Nature. Developing an understanding of its properties is thus critical to
revealing basic features of the Standard Model. We describe experimental
progress in this direction, made using electromagnetic probes, highlighting
both improvements in the precision of charged-pion form factor data, achieved
in the past decade, and new results on the neutral-pion transition form factor.
Both challenge existing notions of pion structure. We also provide a
theoretical context for these empirical advances, first explaining how DCSB
works to guarantee that the pion is unnaturally light; but also, nevertheless,
ensures the pion is key to revealing the mechanisms that generate nearly all
the mass of hadrons. Our discussion unifies the charged-pion elastic and
neutral-pion transition form factors, and the pion's twist-2 parton
distribution amplitude. It also indicates how studies of the charged-kaon form
factor can provide significant contributions. Importantly, recent predictions
for the large-$Q^2$ behaviour of the pion form factor can be tested by
experiments planned at JLab 12. Those experiments will extend precise
charged-pion form factor data to momenta that can potentially serve in
validating factorisation theorems in QCD, exposing the transition between the
nonperturbative and perturbative domains, and thereby reaching a goal that has
long driven hadro-particle physics.
