The concept of quasiparticles is fundamental to quantum many-body physics and to our understanding of the electronic properties of elemental metals. However, there is considerable debate about the stability and quantum numbers (and even the existence) of quasiparticles in a wide range of strongly correlated electron materials. Prominent examples include transition metal oxides, high-Tc cuprate superconductors, and heavy fermion compounds.The frequency dependent optical conductivity has proven to be a powerful experimental probe in addressing these issues. I will describe a recent combined theoretical and experimental study of the low energy electrodynamics of charge carriers close to the Mott metal-insulator transition in a quasi two-dimensional organic charge-transfer salt . The frequency dependence of the conductivity deviates significantly from the Drude model behavior, characteristic of elemental metals. There is a strong redistribution of spectral weight as the Mott transition is approached and with temperature. The quasiparticles only exist at relatively low energies and their effective mass increases considerably near the insulating phase. A dynamical mean-field-theory treatment of the relevant Hubbard model gives a good quantitative description of the experimental data .
 R.H. McKenzie, Nature Physics 3, 756 (2007).
 J. Merino, M. Dumm, N. Drichko, M. Dressel, and R. H. McKenzie,
Phys. Rev. Lett. 100, 086404:1-4 (2008).