Adatoms on semiconductors are inducing surface states and surface dipoles. These adatom-induced surface states are responsible for the pinning of the Fermi level at adsorbate-covered semiconductor surfaces. Adatom-induced surface dipoles, on the other hand, are changing the ionization energy of the surface. Metal atoms evaporated on, for example, GaAs(110) surfaces held at low temperatures were observed to induce surface states of donor character while with nonmetal adatoms such as chlorine, sulfur, and oxygen, on the other hand, adatom-induced surface acceptors were observed. The energy levels of the metal-induced surface donors were found to be linearly correlated with the first ionization energies of the metal atoms. For metal adatoms with one outer s electron, this chemical trend is explained in a surface–molecule or bond picture by using a simple tight-binding approach. In the band picture, on the other hand, adatom-induced surface dipoles are described by the tails of the electron wave functions of the adatoms into the semiconductor. These tails are derived from the virtual gap states of the complex band structure of the semiconductor.