Strongly correlated electron systems have often low-energy excitations in the form of heavy quasiparticles. Their thermodynamic low-temperature properties are similar to those of ordinary Fermi liquids but with a large enhanced mass of the charge carriers. The question arises what the microscopic origin is of those heavy quasiparticles. While originally it was believed that the Kondo effect is the source of such behavior it has become clear that a number of different microscopic processes may result in heavy quasiparticles. They will be discussed in the talk. In particular it will be demonstrated that charge order, strong intra-atomic correlations in 5f systems as well as frustration lead to low energy excitations which show experimental features partially resembling those of the Kondo effect. We want also to discuss the breakdown of the quasiparticle concept.