Present-day understanding of the nature of formation of vibrational spectra of adsorbates is examined. Particular attention is paid to the theoretical side of the problem. A comparison is made of the relative roles of T$_1$ (dissipative damping) and T$_2$ (pure dephasing) processes, and also of electron-hole pairs, phonons and photons in T$_1$ processes. The problem is discussed of separating out the mechanism dominating the determination of line shape. It is noted that inhomogeneous broadening which undoubtedly plays an important role in many cases has received little theoretical attention. An analysis is given of recent experimental data on the basis of modern theoretical concepts.