Abstract The X-ray diffraction experiment of iron at temperatures up to 1000°C, which Albert Hull conducted 100 years ago, in 1917, may be regarded as the first in situ diffraction experiment. Ever since, diffraction methods matured and became widely used and powerful tools for materials characterization and structure determination. Considerable progress was made in radiation source brilliance and diffraction instrumentation, enabling time-dependent in situ studies of a wide range of compounds and processes today. In this contribution, we will give a brief historical sketch of the first in situ diffraction experiment and present some modern-day examples, highlighting the impact of this investigation technique to solid-state sciences.