In this thesis the interaction between a single quantum dot and a cavity is investigated. Quantum dots embedded in a micropillar cavity are investigated by measuring the spectra, and by performing time-resolved measurements. The enhancement of the spontaneous decay of the quantum dot is measured to 18.9 with a resonant lifetime of 28.1 ps under the assumption of an exponential decay. A model is constructed with measurable parameters, and the calculated decay curves are not exponential. As a result the mean decay rate, which coincides with the exponential rate for an exponential decay, is...