  Electromagnetic fluctuation-induced forces between atoms and surfaces are
generally known as Casimir-Polder interactions. The exact knowledge of these
forces is rapidly becoming important in modern experimental set-ups and for
technological applications. Recent theoretical and experimental investigations
have shown that such an interaction is tunable in strength and sign, opening
new perspectives to investigate aspects of quantum field theory and
condensed-matter physics. In this Chapter we review the theory of
fluctuation-induced interactions between atoms and a surface, paying particular
attention to the physical characterization of the system. We also survey some
recent developments concerning the role of temperature, situations out of
thermal equilibrium, and measurements involving ultra-cold atoms.
