  We report on a spectroscopic determination of the stellar parameters and
chemical abundances for the parent star of the transiting planet TrES-1. Based
on a detailed analysis of iron lines in our Keck and HET spectra we derive
$T_\mathrm{eff} = 5250\pm 75$ K, $\log g = 4.6\pm 0.2$, and [Fe/H] $= 0.00\pm
0.09$. By measuring the \ion{Ca}{2} activity indicator and by putting useful
upper limits on the Li abundance we constrain the age of TrES-1 to be $2.5\pm
1.5$ Gyr. By comparing theoretical stellar evolution models with the
observational parameters we obtain $M_\star = 0.89\pm 0.05$ $M_\odot$, and
$R_\star = 0.83\pm 0.05$ $R_\odot$. Our improved estimates of the stellar
parameters are utilized in a new analysis of the transit photometry of TrES-1
to derive a mass $M_p = 0.76\pm 0.05$ $M_\mathrm{J}$, a radius $R_p =
1.04^{+0.08}_{-0.05}$ $R_\mathrm{J}$, and an inclination $i =
89^\circ.5^{+0.5}_{-1.3}$. The improved planetary mass and radius estimates
provide the grounds for new crucial tests of theoretical models of evolution
and evaporation of irradiated extrasolar giant planets.
