  We study a simple model in which the recently reported 750 GeV diphoton
excess arises from a composite pseudo Nambu-Goldstone boson---hidden
pion---produced by gluon fusion and decaying into two photons. The model only
introduces an extra hidden gauge group at the TeV scale with a vectorlike quark
in the bifundamental representation of the hidden and standard model gauge
groups. We calculate the masses of all the hidden pions and analyze their
experimental signatures and constraints. We find that two colored hidden pions
must be near the current experimental limits, and hence are probed in the near
future. We study physics of would-be stable particles---the composite states
that do not decay purely by the hidden and standard model gauge dynamics---in
detail, including constraints from cosmology. We discuss possible theoretical
structures above the TeV scale, e.g. conformal dynamics and supersymmetry, and
their phenomenological implications. We also discuss an extension of the
minimal model in which there is an extra hidden quark that is singlet under the
standard model and has a mass smaller than the hidden dynamical scale. This
provides two standard model singlet hidden pions that can both be viewed as
diphoton/diboson resonances produced by gluon fusion. We discuss several
scenarios in which these (and other) resonances can be used to explain various
excesses seen in the LHC data.
