The concept of effective mass may be used to describe the motion of a nucleon in the nuclear field, at both positive and negative energies. The anomaly around the Fermi energy associated with an energy-dependent effective mass is due to increased absorption in that region. The overall energy dependence of the single-particle potential is associated with the momentum dependence of the Hartree-Fock field. The phenomenological determination of the effective mass from analyses of elastic scattering and other data is described.