Most prokaryotes including bacteria and archaea form macroscopic, surface-attached, multi-cellular communities known as biofilms. Biofilms constitute the majority of bacterial biomass on earth, representing a fundamental mode of bacterial existence in nature. While biofilms may prove beneficial to eukaryotes in host-associated microbiomes, the formation of pathogenic bacterial biofilms is associated with the establishment of serious, chronic, antibiotic-tolerant infections. My laboratory develops electron cryotomography and correlated imaging methods to study how molecules on the surface of bacterial cells mediate bacterial biofilm formation. In my talk, I will present high-resolution three-dimensional snapshots of bacterial biofilms, which together with atomic structures of cell surface molecules that we have solved, provide insights into how bacterial biofilms are organised at the molecular level. Our work has revealed distinct molecular mechanisms of biofilm formation, which has allowed us to develop potent inhibitors of Pseudomonas aeruginosa biofilms. If time permits, I can also show how we are combining cryo-EM with mass spectrometry to derive detailed chemical information about our target molecules inside biofilms and eukaryotic organisms.