Biomolecular mechanisms and interactions form the basis of biological function and regulation. Elucidating the underlying processes typically relies on a combination of structural characterization and bulk studies to reveal the associated energetics and kinetics. Both approaches, however, come with intrinsic biases, such as a tendency towards the most stable species or the averaging of individual to ensemble behaviour, and neither is capable of visualizing the underlying dynamics. Prof. Kukura will show how mass photometry, single-molecule mass measurement in solution, can be used to fully characterise biomolecular interactions, ranging from simple 1:1 interactions to the assembly of 60-mers in the context of virus-like capsids. He will close by presenting a novel assay that enables mass-resolved observation of single-molecule dynamics in solution with ms temporal resolution, which is used to report the first molecular-level observation of viral capsid assembly.