In the beginning of the 1970's, Wilson developed the concept of a
fully non-perturbative renormalization group transformation. Applied
to the Kondo problem, this numerical renormalization group method 
(NRG) gave for the first time the full crossover from the
high-temperature phase of a free spin to the low-temperature phase of
a completely screened spin. The NRG has been later generalized to a 
variety of quantum impurity problems. The purpose of this talk is to 
give a brief introduction to the NRG method, and to present some
of the recent applications of the method, in particular impurity
models with a coupling to bosonic baths.