The theoretical work on the production of low-mass lepton pairs in relativistic heavy-ion collisions is reviewed. In particular I focus on attempts to understand the observed enhancement of low-mass lepton pairs in terms of modified vector meson properties in hadronic matter. I begin with some general remarks, where I discuss the identification of hadrons in a medium with the corresponding quasiparticle excitations of the many-body system. Various medium effects on the properties of the p meson and results on lepton-pair production in simulations of relativistic heavy-ion collisions including these effects are then examined. Finally I describe recent results on the momentum dependence of the p-meson self energy in matter due to pN interactions and present results for the corresponding production rate of e+e- pairs.