Confining the electric field in cavities can lead to ultra-strong light-matter interaction. This allows to control the excited state dynamics of molecules in unprecedented ways. The luminescence properties are strongly modified and also
chemical reactions might be taylored by appropriate cavity fields [1]. In recent years excited state phenomena of nanostructures and molecules were successfully treated at the level of time-dependent density functional theory (TDDFT). This theory solves the time-dependent Schroedinger equation for many-electron systems based on an exact formalism for the electron density. The electric field is treated classically and enters through the dipole approximation. In order to describe the light-matter interaction in cavities one needs to quantize the electric field and treat matter and fields at the same footing