We review the general primordial perturbation valid at early times in a universe with no new physics and consisting of four regular isocurvature modes in addition to an adiabatic growing mode. The prospects for testing the adiabaticity of the cosmic microwave background (CMB) anisotropy using MAP and PLANCK are then discussed. The role of CMB polarization in constraining the presence of primordial isocurvature modes is investigated. ©2001 American Institute of Physics.