The method of photocrystallography allows short-lived excited states, primarily in organometallic and organic compounds, to be characterized structurally with high spatial resolution using electron diffraction. In addition to a better understanding of excited states in general, this method allows in particular (photo)catalytic processes to be better understood at the atomic level. These experiments are typically carried out as so-called pump-probe experiments. First, the sample is excited with a short laser pulse (pump), and the excited state is then structurally characterized with a short time delay using an extremely short electron pulse (probe). Using the method of ultrafast electron diffraction a temporal resolution in the low femtosecond range can be achieved, analogous to experiments on X-ray free electron lasers (XFELs). A major advantage of UED over corresponding XFEL experiments is, that the sample is significantly less damaged and therefore does not need to be replaced constantly.