Recent mid-infrared absorption (Anappara A. A. et al., Appl. Phys. Lett., 87 (2005) 051105) and electroluminescence (Sapienza L. et al., Phys. Rev. Lett., 100 (2008) 136806) experiments on microcavities embedding quantum wells have shown strong coupling between a cavity photon mode and the transition between two conduction subbands, being the lowest one filled with a dense two-dimensional electron gas. We theoretically investigated the effect of this strong light-matter coupling on intersubband electroluminescence. In a first work (De Liberato S. and Ciuti C., Phys. Rev. B, 77 (2008) 155321), using a cluster factorization method, we derived a closed set of dynamical equations for the quantum well carrier and cavity photon occupation numbers, the correlation between the cavity field and the intersubband polarization, as well as polarization-polarization contributions. Solving the resulting set of equations in the stationary regime, we were able to fully characterize the transport and electroluminescence properties as a function of the applied voltage. We discovered a strong-coupling equivalent of the Purcell effect, that can increase the efficiency of intersubband light emitting devices (normally of the order of 10⁻⁵) of various orders of magnitude. In a second work (De Liberato S. and Ciuti C., arxiv:0802.409) we calculated the spectral function of the electrons inside the microcavity. We were thus able to investigate how the electronic states are modified by the coupling to the microcavity vacuum field and showed that resonant electron tunneling from a narrow-band injector can selectively excite superradiant states and produce ultraefficient polariton electroluminescence.