The Kondo effect is one of the most important quantum many-body effects triggered by condensates made of a heavy impurity and a light fermion. Recently, importance of the Kondo effect in dense QCD has been discussed by regarding a heavy (charm or bottom) quark as the impurity in the light-quark medium, which is referred to as the QCD Kondo effect.
Besides, properties of the Kondo effect were found to be described from a field theoretical approach, e.g., the NJL-type model together with the heavy-quark effective theory. In this talk I summarize the recent development of theoretical study of the QCD Kondo effect. Moreover, I explain the latest results of emergence of the heavy-quark spin polarization induced by the effect in a magnetic field, and show its relation to solid-state physics.