We present full sky microwave maps in five frequency bands (23 to 94 GHz) from the WMAP first year sky survey. Calibration errors are 1 per mode to 1 = 658. The temperature-polarization cross-power spectrum reveals both acoustic features and a large angle correlation from reionization. The optical depth of reionization is T = 0.17 f 0.04, which implies a reionization epoch of tr = 180T;iO Myr (95% CL) after the Big Bang at a redshift of zr = 20:;' (95% CL) for a range of ionization scenarios. This early reionization is incompatible with the presence of a significant warm dark matter density. A best-fit cosmological model to the CMB and other measures of large scale structure works remarkably well with only a few parameters. The age of the best-fit universe is t~ = 13.7 f 0.2 Gyr old. Decoupling was tdec = 379;; kyr after the Big Bang at a redshift of aec = 1089 f 1. The thickness of the decoupling surface was = 195 -f 2. The matter density of the universe is R,h2 = 0.135+$$:, the baryon density is Rbh2 = 0.0224 f0.0009, and the total mass-energy of the universe is R,,, = 1.02 f 0.02. There is progressively less fluctuation power on smaller scales, from W A P to fine scale CMB measurements to galaxies and finally to the Ly-a: forest. This is accounted for with a running spectral index, significant at the N 20 level. The spectral index of scalar fluctuations is fit as n, = 0.93 f0.03 at wavenumber ko = 0.05 Mpc-' (I,,-,w 700), with a slope of dn,/d Ink = -0.031+$$6, in the best-fit model. This flat