The High-z Supernova Search Team has discovered and observed eight new supernovae (SN Ia) in the redshift range of 0.3 to 1.2. These observations, analyzed using similar but distinct methods, confirm the result of previous studies that SN Ia luminosity distances imply an accelerating universe. Importantly, these measurements extend the redshift range of consistently observed SN Ia to approximately 1, where the signature of cosmological effects has the opposite sign of some systematic effects. This provides a quantitative confirmation of dark energy and a qualitative test for the cosmological origin of cosmic acceleration. The team finds a rate for SN Ia of $(1.4 \pm 0.5) \times 10^{-4} \, h^3 \, \text{Mpc}^{-3} \, \text{yr}^{-1}$ at a mean redshift of 0.5. They present distances and host extinctions for 230 SN Ia, which place constraints on cosmological parameters. If the equation of state parameter of dark energy is $w = -1$, then $H_0 t_0 = 0.96 \pm 0.04$, and $\Omega_\Lambda - 1.4 \Omega_M = 0.35 \pm 0.14$. Including the constraint of a flat universe, they find $\Omega_M = 0.28 \pm 0.05$. Adopting a prior based on the 2dF redshift survey, they find that the equation of state parameter of dark energy lies in the range $-1.48 < w < -0.72$ at 95% confidence. These results are consistent with recent findings from the WMAP satellite.The High-z Supernova Search Team has discovered and observed eight new supernovae (SN Ia) in the redshift range of 0.3 to 1.2. These observations, analyzed using similar but distinct methods, confirm the result of previous studies that SN Ia luminosity distances imply an accelerating universe. Importantly, these measurements extend the redshift range of consistently observed SN Ia to approximately 1, where the signature of cosmological effects has the opposite sign of some systematic effects. This provides a quantitative confirmation of dark energy and a qualitative test for the cosmological origin of cosmic acceleration. The team finds a rate for SN Ia of $(1.4 \pm 0.5) \times 10^{-4} \, h^3 \, \text{Mpc}^{-3} \, \text{yr}^{-1}$ at a mean redshift of 0.5. They present distances and host extinctions for 230 SN Ia, which place constraints on cosmological parameters. If the equation of state parameter of dark energy is $w = -1$, then $H_0 t_0 = 0.96 \pm 0.04$, and $\Omega_\Lambda - 1.4 \Omega_M = 0.35 \pm 0.14$. Including the constraint of a flat universe, they find $\Omega_M = 0.28 \pm 0.05$. Adopting a prior based on the 2dF redshift survey, they find that the equation of state parameter of dark energy lies in the range $-1.48 < w < -0.72$ at 95% confidence. These results are consistent with recent findings from the WMAP satellite.