Experimental and theoretical research on nonadiabatic effects in metals is briefly reviewed. It is shown that these effects are most manifest in optical phonon spectra at small wave vectors q satisfying the condition qvF < ω0(q), where vF is the electron Fermi velocity, and ω0(q) is the phonon frequency. Various theoretical approaches to calculating nonadiabatic effects are examined. Frohlich's phenomenological field-theoretical model, the most widely used in such calculations, is discussed in detail, some aspects of its application are considered, and its efficiency as a tool for studying nonadiabatic effects is demonstrated.