We present full-sky microwave maps in five frequency bands (23-94 GHz) from the Wilkinson Microwave Anisotropy Probe (WMAP) first-year sky survey. Calibration errors are less than 0.5%, and the low systematic error level is well specified. The cosmic microwave background (CMB) is separated from the foregrounds using multifrequency data. The sky maps are consistent with the 7° FWHM Cosmic Background Explorer (COBE) maps. We report more precise, but consistent, dipole and quadrupole values. The CMB anisotropy obeys Gaussian statistics with -58 < fNL < 134 (95% confidence level [CL]). The 2 ≤ ℓ ≤ 900 anisotropy power spectrum is cosmic-variance-limited for ℓ < 354, with a signal-to-noise ratio greater than 1 per mode to ℓ = 658. The temperature-polarization cross-power spectrum reveals both acoustic features and a large-angle correlation from reionization. The optical depth of reionization is τ = 0.17 ± 0.04, which implies a reionization epoch of tr = 180 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 t0 = 13.7 ± 0.2 Gyr. Decoupling was tdec = 379 kyr after the big bang at a redshift of zdec = 1089 ± 1. The thickness of the decoupling surface was Δzdec = 195 ± 2. The matter density of the universe is Ωmh2 = 0.135, the baryon density is Ωbh2 = 0.0224 ± 0.0009, and the total mass-energy of the universe is Ωtot = 1.02 ± 0.02. It appears that there may be progressively less fluctuation power on smaller scales, from WMAP to fine-scale CMB measurements to galaxies and finally to the Lyα forest. This may be accounted for with a running spectral index of scalar fluctuations, fitted as ns = 0.93 ± 0.03 at wavenumber k0 = 0.05 Mpc-1 (ℓeff ≈ 700), with a slope of dns/d ln k = -0.031 in the best-fit model. (For WMAP data alone, ns = 0.99 ± 0.04.) This flat universe model is composed of 4.4% baryons, 22% dark matter, and 73% dark energy. The dark energy equation of state is limited to w < -0.78 (95% CL). Inflation theory is supported with ns ≈ 1, Ωtot ≈ 1, Gaussian random phases of the CMB anisotropy, and superhorizon fluctuations implied by the temperature-polarization anticorrelations at decoupling. An admixture of isocurvature modes does not improve the fit. The tensor-to-scalar ratio is r(k0 = 0.002 Mpc-1) < 0.90 (95% CL). The lack of CMB fluctuation power on the largest angular scales reported by COBE and confirmed by WMAP is intriguing. WMAP continues to operate, so results will improve.