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We present the angular power spectrum derived from the first-year Wilkinson Microwave Anisotropy Probe (WMAP) sky maps. We study a variety of power spectrum estimation methods and data combinations and demonstrate that the results are robust. The data are modestly contaminated by diffuse Galactic foreground emission, but we show that a simple Galactic template model is sufficient to remove the signal. Point sources produce a modest contamination in the low-frequency data. After masking∼ 700 known bright sources from the maps, we estimate that residual sources contribute∼ 3500 μ K 2 at 41 GHz and∼ 130 μ K 2 at 94 GHz to the power spectrum [ℓ (ℓ+ 1) C ℓ/2 π] at ℓ= 1000. Systematic errors are negligible compared to the (modest) level of foreground emission. Our best estimate of the power spectrum is derived from 28 cross-power spectra of statistically independent channels. The final spectrum is …