Chiral transitions in QCD at finite temperature and density are discussed within a composite operator formalism. For massless quarks the phase diagram in temperature and chemical potentials presents a tricritical point at the intersection of the critical line for first-order phase transitions and second-order transitions. The overall picture is not sensibly affected by small quark masses, except that the quark condensate no longer vanishes for large temperatures and/or chemical potentials, and for first-order transitions it remains discontinuous only for small quark mass values. Such effects are discussed by Clausius-Clapeyron-like relations. The Landau expansion is used around the tricritical point and for second-order transitions.