This PhD thesis is concerned with the chiral phase transition of QCD with two degenerate light quark masses and a strange quark mass close to its physical value. We analyze the quark mass dependence of the chiral condensate and chiral susceptibilities close to the transition temperature. The analysis is twofold:

First we provide evidence for the influence of thermal fluctuations of Goldstone modes on the chiral condensate at finite temperature. We show that at temperatures below but close to the chiral phase transition at vanishing quark mass this leads to a characteristic dependence of the light quark chiral condensate on the square root of the light quark mass. As a consequence the chiral susceptibility shows a strong quark mass dependence for all temperatures below the critical temperature and diverges like the inverse square root of the light quark mass in the chiral limit. We separately examine the divergence of disconnected and connected parts of the light quark susceptibility and discuss the volume as well as cut-off dependence of susceptibilities and chiral condensates.

Second we analyze the critical behavior of the chiral transition with a scaling analysis based on the O(N) scaling functions. We find strong evidence for 2nd order O(N) scaling in the chiral limit of the light quark mass and with physical strange quark mass. Z(2) scaling is disfavored for finite values of a critical light quark mass, which indicates that the physical strange quark mass is above the tricritical mass. The scaling fits are based on the magnetic equation of state for the chiral condensate. We compare these fit results also with the corresponding scaling functions for the chiral susceptibilities and identify the Goldstone contributions and attempt to identify the connected and disconnected susceptibility contributions. We discuss the deviations from scaling and compare results for two different lattice spacings. Finally we present the result on the pseudocritical line for zero chemical potential and the curvature of the critical line for non-zero chemical potential to lowest order.