Celestial objects are often home to complex, dynamic, intriguing environments. High‐resolution x‐ray spectra from these sources measured by satellites such as the Chandra, XMM‐Newton, the Solar Maximum Mission, and the soon‐to‐be‐launched Astro‐E2 provide a means for understanding the physics governing these sources. Especially rich is the x‐ray emission from L‐shell transitions in highly charged iron ions. This emission is the source of a variety of diagnostics whose utility lies in the accuracy of the atomic data employed to model the x‐ray spectra. The atomic data used to describe these diagnostics are generally provided by large theoretical calculations and benchmarked by laboratory data. In this paper we discuss laboratory measurements of Fe L‐shell x‐ray emission including wavelengths, relative and absolute excitation cross sections, and line ratios that provide diagnostics of temperature and density.