Quantum optics and its applications, like quantum key distribution, have a high potential of becoming a significant technology in the future. The full potential of this technology is still far from reached and additional knowledge about the quantum mechanical behaviour of photons from different sources interacting at beamsplitters is essential. In this context the Hong-Ou-Mandel effect is maybe the most famous example. The Hong-Ou-Mandel effect has been observed many times between photons originating from the same type of source and has already become a standard for characterising photon sources. Relatively new is its observation between two fundamentally different sources. This book describes an experiment in which the Hong-Ou-Mandel effect is realized between photons originating from a quantum dot and from spontaneous parametric down-conversion. The photons from the down-conversion are spectrally filtered in order to match the quantum dot photons by using two different methods. This allows determining the dominant limiting factors for Hong-Ou-Mandel experiments between photons from these two sources.