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We report on the cosmological parameters derived from observations with the Cosmic Background Imager (CBI), covering 40 deg 2 and the multipole range 300≲ l≲ 3500. The angular scales probed by the CBI correspond to structures that cover the mass range from 10 14 to 10 17 M⊙, and the observations reveal, for the first time, the seeds that gave rise to clusters of galaxies. These unique, high-resolution observations also show damping in the power spectrum to l∼ 2000, which we interpret as being due to the finite width of the photon-baryon decoupling region and the viscosity operating at decoupling. Because the observations extend to much higher l, the CBI results provide information complementary to that probed by the BOOMERANG, DASI, MAXIMA, and VSA experiments. When the CBI observations are used in combination with those from COBE-DMR, we find evidence for a flat universe, Ω tot= 1.00 (1 σ …