The High-z Supernova Search Team has discovered and observed 8 new supernovae in the redshift interval z = 0.3 - 1.2. These observations confirm the result that supernova luminosity distances imply an accelerating universe. They extend the redshift range of consistently observed SN Ia to z ≈ 1, where the signature of cosmological effects has the opposite sign of some plausible systematic effects. These measurements provide another quantitative confirmation of the importance of dark energy and constitute a powerful qualitative test for the cosmological origin of cosmic acceleration. The team found a rate for SN Ia of (1.4 ± 0.5) × 10⁻⁴ h³ Mpc⁻³ yr⁻¹ at a mean redshift of 0.5. They present distances and host extinctions for 230 SN Ia, placing constraints on cosmological quantities. If the equation of state parameter of dark energy is w = -1, then H₀t₀ = 0.96 ± 0.04, and ΩΛ - 1.4ΩM = 0.35 ± 0.14. Including the constraint of a flat universe, they find ΩM = 0.28 ± 0.05. Adopting a prior based on the 2dF redshift survey constraint on ΩM and assuming a flat universe, they find that the equation of state parameter of dark energy lies in the range -1.48 < w < -0.72 at 95% confidence. If w > -1, they find w < -0.73 at 95% confidence. These constraints are similar in precision and value to recent results from the WMAP satellite. The paper describes the search, shows spectra of the supernovae, and provides photometric results. It also gives an account of the analysis, including K-corrections, fits to the light curves, and luminosity distances. The paper discusses the inferred cosmological parameters and how these results can be used to test for systematic errors in assessing cosmic acceleration from SN Ia observations. The paper also describes the photometric calibrations and reductions, the properties of the host galaxies, and the photometry of SN Ia. The results show that the universe is dominated by dark energy, and that the cosmological constant or other forms of dark energy are responsible for the observed cosmic acceleration. The paper also discusses the implications of these results for theoretical physics and the need for further supernova data to confirm cosmic acceleration.The High-z Supernova Search Team has discovered and observed 8 new supernovae in the redshift interval z = 0.3 - 1.2. These observations confirm the result that supernova luminosity distances imply an accelerating universe. They extend the redshift range of consistently observed SN Ia to z ≈ 1, where the signature of cosmological effects has the opposite sign of some plausible systematic effects. These measurements provide another quantitative confirmation of the importance of dark energy and constitute a powerful qualitative test for the cosmological origin of cosmic acceleration. The team found a rate for SN Ia of (1.4 ± 0.5) × 10⁻⁴ h³ Mpc⁻³ yr⁻¹ at a mean redshift of 0.5. They present distances and host extinctions for 230 SN Ia, placing constraints on cosmological quantities. If the equation of state parameter of dark energy is w = -1, then H₀t₀ = 0.96 ± 0.04, and ΩΛ - 1.4ΩM = 0.35 ± 0.14. Including the constraint of a flat universe, they find ΩM = 0.28 ± 0.05. Adopting a prior based on the 2dF redshift survey constraint on ΩM and assuming a flat universe, they find that the equation of state parameter of dark energy lies in the range -1.48 < w < -0.72 at 95% confidence. If w > -1, they find w < -0.73 at 95% confidence. These constraints are similar in precision and value to recent results from the WMAP satellite. The paper describes the search, shows spectra of the supernovae, and provides photometric results. It also gives an account of the analysis, including K-corrections, fits to the light curves, and luminosity distances. The paper discusses the inferred cosmological parameters and how these results can be used to test for systematic errors in assessing cosmic acceleration from SN Ia observations. The paper also describes the photometric calibrations and reductions, the properties of the host galaxies, and the photometry of SN Ia. The results show that the universe is dominated by dark energy, and that the cosmological constant or other forms of dark energy are responsible for the observed cosmic acceleration. The paper also discusses the implications of these results for theoretical physics and the need for further supernova data to confirm cosmic acceleration.