X-ray free electron lasers (XFEL) provide a billion-fold jump in the peak X-ray brilliance when compared with synchrotron radiation. One area where XFEL radiation is having an impact is time-resolved structural studies of protein conformational changes. I will describe how we used time resolved serial femtosecond crystallography at an XFEL to probe light-driven structural changes in bacteriorhodopsin. Bacteriorhodopsin is a light-driven proton pump which has long been used as a model system in biophysics. The mechanism by which light-driven isomerization of a retinal chromophore is coupled to the transport of protons “up-hill” against a transmembrane proton concentration gradient involves protein structural changes. Collaborative studies performed at SACLA (An XFEL in Japan) have probed structural changes in microcrystals on a time-scale from nanoseconds to milliseconds. Structural results from these studies enabled a complete picture of structural changes occurring during proton pumping by bacteriorhodopsin to be recovered (Nango et al., 2016).