Observations of the cosmic microwave background will be an important ingredient in our efforts to understand the epoch of galaxy formation. Microwave background observations are most sensitive to physical conditions near the surface of last scatter,z∼1100.MAP should enable cosmologists to infer many of the initial conditions for galaxy formation. The microwave background photons also illuminate the “dark age”. Measurements of microwave background polarization can determine or at least constrain the redshift at which stars (or quasars) first reionize the universe. The observations are also sensitive to non-linear density fluctuations during this epoch. Microwave background measurements also probe the nearby universe. Low redshift objects produce additional fluctuations through gravitational lensing, the ISW effect and the Sunyaev-Zeldovich effect. ©1999 American Institute of Physics.