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 (photo)catalytic processes in particular 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 this 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 applied here, time resolutions 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.