  Pseudoscalars appearing in particle physics are reviewd systematically. From
the fundamental point of view at an ultra-violat completed theory, they can be
light if they are realized as pseudo-Goldstone bosons of some spontaneously
broken global symmetries. The spontaneous breaking scale is parametrized by the
decay constant $f$. The global symmetry is defined by the lowest order terms
allowed in the effective theory consistent with the gauge symmetry in question.
Since any global symmetry is known to be broken at least by quantum
gravitational effects, all pseudoscalars should be massive. The mass scale is
determined by $f$ and the explicit breaking terms $\Delta V$ in the effective
potential and also anomaly terms $\Delta\Lambda^4_G$ for some non-Abelian gauge
groups $G$. The well-known example by non-Abelian gauge group breaking is the
potential for the "invisible" QCD axion, via the Peccei-Quinn symmetry, which
constitutes a major part of this review. Even if there is no breaking terms
from gauge anomalies, there can be explicit breaking terms $\Delta V$ in the
potential in which case the leading term suppressed by $f$ determines the
pseudoscalar mass scale. If the breaking term is extremely small and the decay
constant is trans-Planckian, the corresponding pseudoscalar can be a candidate
for a `quintessential axion'. In general, $(\Delta V)^{1/4}$ is considered to
be smaller than $f$, and hence the pseudo-Goldstone boson mass scales are
considered to be smaller than the decay constants. In such a case, the
potential of the pseudo-Goldstone boson at the grand unification scale is
sufficiently flat near the top of the potential that it can be a good candidate
for an inflationary model, which is known as `natural inflation'. We review all
these ideas in the bosonic collective motion framework.

High Energy Physics - Phenomenology