Recent experimental data and theoretical developments on the production of lepton pairs in hadron collisions are reviewed. With emphasis on the interplay between theory and experiment, the relevance of theoretical calculations to the data available at present energies is critically examined. The Drell--Yan mechanism is found to be phenomenologically dominant provided that the parton distribution functions contain effects of gluon radiation in a narrow cone. Explicit QCD perturbative calculations of the non-Drell--Yan type yield results that are apparently important at large transverse momenta, but are contradicated by subsequent data at 400 GeV and below. A consistent picture in the parton model is sketched. Further experiments to probe the basic mechanism are suggested. 83 references.