AbstractX‐ray diffraction with femtosecond time‐resolution represents a direct probe of ultrafast structural changes in condensed matter. The generation of ultrashort X‐ray pulses in laser‐driven plasma and/or accelerator‐based sources has made substantial progress, and has allowed for studies of transient structures with an unprecedented accuracy. Herein, recent work on transient crystalline structures is reviewed, with the focus on laser‐based experiments.