Transition temperature in QCD

We present a detailed calculation of the transition temperature in QCD with two light and one heavier (strange) quark mass on lattices with temporal extent N_=4 and 6. Calculations with improved staggered fermions have been performed for various light to strange quark mass ratios in the range, 0. 05^{m}₋/^{m}ₒ0. 5, and with a strange quark mass fixed close to its physical value. From a combined extrapolation to the chiral (^{m}₋0) and continuum (aT1/N_0) limits we find for the transition temperature at the physical point T₂r₀=0. 457 (7) where the scale is set by the Sommer-scale parameter r₀ defined as the distance in the static quark potential at which the slope takes on the value, (dVₐₐ (r) /dr) ₑ=ₑ₀=1. 65/r₀^2. Using the currently best known value for r₀ this translates to a transition temperature T₂=192 (7) (4) MeV. The transition temperature in the chiral limit is about 3% smaller. We discuss current ambiguities in the determination of T₂ in physical units and also comment on the universal scaling behavior of thermodynamic quantities in the chiral limit.