  The 2.725 K cosmic microwave background has played a key role in the
development of modern cosmology by providing a solid observational foundation
for constraining possible theories of what happened at very large redshifts and
theoretical speculation reaching back almost to the would-be big bang initial
singularity. After recounting some of the lesser known history of this area, I
summarize the current observational situation and also discuss some exciting
challenges that lie ahead: the search for B modes, the precision mapping of the
CMB gravitational lensing potential, and the ultra-precise characterization of
the CMB frequency spectrum, which would allow the exploitation of spectral
distortions to probe new physics.
