A detailed mechanistic understanding of how a protein functions requires knowledge not only of its static structure, but also how its conformation evolves as it executes its function. The recent development of picosecond time-resolved X-ray crystallography has allowed us to visualize in real time and with atomic detail the conformational evolution of a protein. Here, we report the photolysis-induced structural evolution of wild-type and L29F myoglobin over times ranging from 100 ps to 3 mus. The sub-ns structural rearrangements that accompany ligand dissociation in wild-type and the mutant form differ dramatically, and lead to vastly different ligand migration dynamics. The correlated protein displacements provide a structural explanation for the kinetic differences. Our observation of functionally important protein motion on the sub-ns time scale was made possible by the 150-ps time resolution of the measurement, and demonstrates that picosecond dynamics are relevant to protein function. To visualize subtle structural changes without modeling, we developed a novel method for rendering time-resolved electron density that depicts motion as a color gradient across the atom or group of atoms that move. A sequence of these time-resolved images have been stitched together into a movie, which allows one to literally "watch" the protein as it executes its function. Published by Elsevier Inc.