  The frequency spectrum of the cosmic microwave background (CMB) is a
relatively untapped source of data which can allow us to peer beyond the
surface of last scattering. Small deviations away from a perfect blackbody
shape will encode valuable information about the state of the primordial
Universe which may not be accessible by other means. Here, we briefly review
some key science goals of CMB spectral distortions, with an emphasis on how
future generations of experiments can be used in tandem with complementary
observational probes to perform model discrimination of exotic physics
scenarios. We focus here on synergies between spectral distortions,
gravitational waves, and 21cm cosmology.
