Abstract: The biological functions of proteins are highly related to their structures and dynamics. The determination of protein structures at atomic resolution has advanced our understanding of the underlying molecular mechanisms, thanks to the development of X- ray diffraction and electron microscopy techniques, especially facilitated by computational algorithms and software. However, most studies are based on static structures, as it is challenging to probe the dynamics of proteins, i.e., the dynamical structures of “molecules-in-action”. The X-ray free electron lasers generate ultrabright femtosecond pulses, enabling time-resolved crystallography in “pump-probe” approaches. By determining a series of structures at specific time points after activation, molecular movies can be produced. Only by revealing the detailed dynamics through molecular movies can the mechanisms be fully understood.