The first year of observations from the Wilkinson Microwave Anisotropy Probe (WMAP) provides full sky microwave maps in five frequency bands (23 to 94 GHz). The maps show a low level of systematic errors and are consistent with the Cosmic Background Explorer (COBE) maps. The cosmic microwave background (CMB) anisotropy is found to obey Gaussian statistics with a non-Gaussian parameter $ f_{NL} $ in the range -58 to 134 (95% CL). The anisotropy power spectrum is cosmic variance limited for $ l < 354 $, with a signal-to-noise ratio greater than 1 per mode. The temperature-polarization cross-power spectrum reveals acoustic features and a large angle correlation from reionization. The optical depth of reionization is $ \tau = 0.17 \pm 0.04 $, implying a reionization epoch of $ t_r = 180_{-80}^{+220} $ Myr after the Big Bang at a redshift of $ z_r = 20_{-9}^{+10} $ (95% CL). This early reionization is incompatible with the presence of significant warm dark matter density. A best-fit cosmological model to the CMB and other large-scale structure measurements works remarkably well with only a few parameters. The age of the best-fit universe is $ t_0 = 13.7 \pm 0.2 $ Gyr. Decoupling occurred $ t_{dec} = 379_{-7}^{+8} $ kyr after the Big Bang at a redshift of $ z_{dec} = 1089 \pm 1 $. The thickness of the decoupling surface was $ \Delta z_{dec} = 195 \pm 2 $. The matter density of the universe is $ \Omega_m h^2 = 0.135_{-0.009}^{+0.008} $, the baryon density is $ \Omega_b h^2 = 0.0224 \pm 0.0009 $, and the total mass-energy of the universe is $ \Omega_{tot} = 1.02 \pm 0.02 $. There may be progressively less fluctuation power on smaller scales, from WMAP to fine-scale CMB measurements to galaxies and finally to the Ly-α forest. This may be accounted for with a running spectral index of scalar fluctuations, fit as $ n_s = 0.93 \pm 0.03 $ at $ k_0 = 0.05 $ Mpc$^{-1}$ ( $ l_{eff} \approx 700 $ ), with a slope ofThe first year of observations from the Wilkinson Microwave Anisotropy Probe (WMAP) provides full sky microwave maps in five frequency bands (23 to 94 GHz). The maps show a low level of systematic errors and are consistent with the Cosmic Background Explorer (COBE) maps. The cosmic microwave background (CMB) anisotropy is found to obey Gaussian statistics with a non-Gaussian parameter $ f_{NL} $ in the range -58 to 134 (95% CL). The anisotropy power spectrum is cosmic variance limited for $ l < 354 $, with a signal-to-noise ratio greater than 1 per mode. The temperature-polarization cross-power spectrum reveals acoustic features and a large angle correlation from reionization. The optical depth of reionization is $ \tau = 0.17 \pm 0.04 $, implying a reionization epoch of $ t_r = 180_{-80}^{+220} $ Myr after the Big Bang at a redshift of $ z_r = 20_{-9}^{+10} $ (95% CL). This early reionization is incompatible with the presence of significant warm dark matter density. A best-fit cosmological model to the CMB and other large-scale structure measurements works remarkably well with only a few parameters. The age of the best-fit universe is $ t_0 = 13.7 \pm 0.2 $ Gyr. Decoupling occurred $ t_{dec} = 379_{-7}^{+8} $ kyr after the Big Bang at a redshift of $ z_{dec} = 1089 \pm 1 $. The thickness of the decoupling surface was $ \Delta z_{dec} = 195 \pm 2 $. The matter density of the universe is $ \Omega_m h^2 = 0.135_{-0.009}^{+0.008} $, the baryon density is $ \Omega_b h^2 = 0.0224 \pm 0.0009 $, and the total mass-energy of the universe is $ \Omega_{tot} = 1.02 \pm 0.02 $. There may be progressively less fluctuation power on smaller scales, from WMAP to fine-scale CMB measurements to galaxies and finally to the Ly-α forest. This may be accounted for with a running spectral index of scalar fluctuations, fit as $ n_s = 0.93 \pm 0.03 $ at $ k_0 = 0.05 $ Mpc$^{-1}$ ( $ l_{eff} \approx 700 $ ), with a slope of