The Kondo effect refers to the non-trivial physics associated with the presence of a magnetic impurity in a solid (generally, a metal). It was found that the resistance of a metal with these impurities does not decrease to a constant at low temperatures (as for a metal with non-magnetic impurities), but actually hits a shallow minimum at a temperature of order 10 K, then increases at lower temperatures. Kondo did the first proper calculation regarding this effect, which showed that in higher orders of perturbation theory, the resistance will diverge as the emperature approaches 0 K. Later calculations refined this result to produce a finite resistivity but retained the feature of a resistance minimum at a non-zero (so-called Kondo) temperature. The Anderson model and accompanying renormalization theory was an important contribution to understanding the underlying physics of the problem.