The focus of this thesis is the light-mediated coupling in planar photonic resonators using the Finite-Difference Time-Domain (FDTD) method. With this method, the three-dimensional Maxwells equations are evaluated numerically. Two different kinds of optical resonators are investigated: microdisk resonators and photonic crystal cavities. Eigenmodes of microdisk resonators under the influence of shape changes are invesitgated. The usage of an uniaxial anisotropic environment, in which a microdisk is embedded, shows anticrossings in the spectral response between eigenmodes of different symmetry, which couple via the environment. Experimental data of a microdisk, embedded in a liquid crystal for dynamic spectral tuning, are confirmed by numerical simulation with the usage of an uniaxial anisotropic environment. A photonic crystal cavity with a line defect, consisting of 7 missing air holes, is spectrally modified via a thin additional layer, added on one side of the slab. A comparison with experimental data shows good agreement. Strong interaction between microdisk resonators and between photonic crystal cavities, aligned in different geometries, are investigated numerically. Distant-dependent, strongly asymmetric splitting in frequency and cavity decay time occurs and is explained. The resonant coupling of a semiconductor quantum dot, where the optical polarization field is calculated via dynamical, nonlinear equations of motion on a quantum mechanical level, on a single mode of a photonic crystal cavity under the influence of an intense laser field is investigated. Thereby, Q-factor dependent side bands occur.