The paper presents the first-year observations and results from the Wilkinson Microwave Anisotropy Probe (WMAP), a NASA mission designed to study the cosmic microwave background (CMB) radiation. The WMAP team reports full-sky microwave maps in five frequency bands (23 to 94 GHz), with calibration errors less than 0.5% and low systematic errors. The CMB is separated from foreground emissions using multifrequency data, and the maps are consistent with the COBE maps. The CMB anisotropy follows Gaussian statistics, and the angular power spectrum is cosmic variance limited for $l < 354$ with a signal-to-noise ratio >1 per mode to $l = 658$. The temperature-polarization cross-power spectrum reveals acoustic features and a large-angle correlation from reionization, with an optical depth of $\tau = 0.17 \pm 0.04$. The best-fit cosmological model to the CMB and other large-scale structure measurements is a flat universe composed of 4.4% baryons, 22% dark matter, and 73% dark energy. The age of the universe is $t_0 = 13.7 \pm 0.2$ Gyr, and the matter density is $\Omega_m h^2 = 0.135^{+0.008}_{-0.009}$. The paper also discusses foreground analyses, limits on non-Gaussianity, and the detection of the reionization of the universe.The paper presents the first-year observations and results from the Wilkinson Microwave Anisotropy Probe (WMAP), a NASA mission designed to study the cosmic microwave background (CMB) radiation. The WMAP team reports full-sky microwave maps in five frequency bands (23 to 94 GHz), with calibration errors less than 0.5% and low systematic errors. The CMB is separated from foreground emissions using multifrequency data, and the maps are consistent with the COBE maps. The CMB anisotropy follows Gaussian statistics, and the angular power spectrum is cosmic variance limited for $l < 354$ with a signal-to-noise ratio >1 per mode to $l = 658$. The temperature-polarization cross-power spectrum reveals acoustic features and a large-angle correlation from reionization, with an optical depth of $\tau = 0.17 \pm 0.04$. The best-fit cosmological model to the CMB and other large-scale structure measurements is a flat universe composed of 4.4% baryons, 22% dark matter, and 73% dark energy. The age of the universe is $t_0 = 13.7 \pm 0.2$ Gyr, and the matter density is $\Omega_m h^2 = 0.135^{+0.008}_{-0.009}$. The paper also discusses foreground analyses, limits on non-Gaussianity, and the detection of the reionization of the universe.