  We determine the second order endpoint of the line of first order phase
transitions, which occur in the light quark mass regime of 3-flavour QCD at
finite temperature, and analyze universal properties of this chiral critical
point. A detailed analysis of Binder cumulants and the joint probability
distributions of energy like and ordering-field like observables confirms that
the chiral critical point belongs to the universality class of the three
dimensional Ising model. From a calculation with improved gauge and staggered
fermion actions we estimate that the transition is first order for
pseudo-scalar meson masses less than about 200 MeV.
