The phase transition from quark-gluon plasma to hadronic matter is extensively examined in relation to the hadronization problem in the hydrodynamic theory of multiple production. Existing models of hadrons and thermalization mechanisms in ultrarelativistic collisions are briefly discussed. When surface interaction effects are taken into account, the phase transition is nontrivial in the bag model: metastable states of matter are possible. Possible hadronization scenarios are discussed, and a kinetic analysis is given of cooling and hadronization processes. It is shown that, when the initial plasma energy density is close to the critical value ($\sim$4 GeV/fm$^3$), the more realistic scenarios are those based on the nonequilibrium hadronization of supercooled plasma, which involves an appreciable increase in the mean transverse momentum of secondary particles. Traditional (equilibrium) mechanisms should predominate in hotter plasmas. Theoretical estimates are compared with JACEE cosmic-ray data.