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-tau=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 boolean AND(l)/m boolean AND(s)<= 0.5, and with a strange quark mass fixed close to its physical value. From a combined extrapolation to the chiral (m boolean AND(l)-> 0) and continuum (aT equivalent to 1/N-tau -> 0) limits we find for the transition temperature at the physical point T(c)r(0)=0.457(7) where the scale is set by the Sommer-scale parameter r(0) defined as the distance in the static quark potential at which the slope takes on the value, (dV(q ($) over barq)(r)/dr)(r=r0)=1.65/r(0)(2). Using the currently best known value for r(0) this translates to a transition temperature T-c=192(7)(4) MeV. The transition temperature in the chiral limit is about 3% smaller. We discuss current ambiguities in the determination of T-c in physical units and also comment on the universal scaling behavior of thermodynamic quantities in the chiral limit.