This chapter discusses defect-solute interaction and complexes formed when a vacancy or self-interstitial becomes trapped by solute atoms (SA) and forms a vacancy-solute atom (V-SA) or self-interstitial atom-solute atom (SIA-SA) complex. The only materials to gain a quantitative understanding are the simple metals, where pseudopotential theory enables to construct interaction potentials. V-SA interactions as well as a whole range of SIA-SA complexes in Al were studied, however, the accuracy of these calculations is not sufficient to predict with certainty the exact SIA-SA structure. A qualitative understanding of the properties of SIA-SA complexes in the fcc lattice can be gained by a simple model in which the host interaction is described e.g. by a Morse potential. The solute-host interaction is described by the pure metal potential shifted by a small amount to smaller or larger distances. In addition to the experimental methods available for the investigations of SIA and vacancies nuclear methods are available for the investigations of special SIA-SA and V-SA complexes. In addition to the binding energy and the migration energy of the SIA-SA complex the trapping radius is an important parameter to describe the SIA-SA interaction. The reaction probability of a migrating SIA with a SA can be expressed in terms of this trapping radius. Nuclear methods have proved to be specially valuable for the investigation of the details of defect-solute interactions in addition to the more general methods.