The scattering of conduction electrons in metals owing to impurities with magnetic moments. This scattering increases the electrical resistance and has the consequence that, in contrast to ordinary metals, the resistance reaches a minimum as the temperature is lowered and then increases as the temperature is lowered further. The Kondo effect is named after Jun Kondo, who analysed this phenomenon in 1964. A fully quantitative description of the Kondo effect is a very difficult many-body problem, requiring techniques such as the renormalization group.