The talk addresses the chiral phase transition of QCD with massless quarks. This is a challenging problem for lattice computations as the chiral limit of QCD is not directly simulable. Its study, however, provides constraints on the phase diagram of QCD with physical masses. We show that the chiral limit is approached via tricritical scaling relations which let us determine the order and temperature of the phase transition as a function of the number of quark flavours $N_f$. Based on simulations of lattice QCD with standard staggered fermions, it was found that QCD with massless quarks has a second order phase transition for $N_f \leq 7$. Additionally, we confirm an expected decrease in the critical temperature for increasing number of flavours. Running simulations on finer lattices and larger $N_f$ will allow us to resolve the question whether massless QCD approaches the conformal window by a first or a second order phase transition.