Resumen: Along this dissertation we review and understand the basic single-atom cavity QED models, as well as analyse their basic phenomenology and the different regimes they exhibit depending on the coupling strength between light and matter. We also establish that cavity QED implementations are many-body systems that are difficult to handle both analytically and numerically, and understand the polaron transform as a useful tool to treat these models semi-analytically. Furthermore, we identify the biased spin-boson model and study its properties via a generalisation of the polaron ansatz, comparing it with respect to the original. Further, we also validate our results against exact diagonalisation and draw some conclusions.