Both Gram-negative and Gram-positive pathogenic bacteria present a remarkable number of surface-exposed organelles and secreted toxins that allow them to control the primary stages of infection, bacterial attachment to host cell receptors and colonization. The mediators of these processes, called adhesins, form a heterogeneous group that varies in architecture, domain content and mechanism of binding. A full understanding of how adhesins mediate cellular adhesion and colonization requires quantitative functional assays to evaluate the strength of the binding interactions, as well as determination of the high-resolution three-dimensional structures of the molecules to provide the atomic details of the interactions. The combination of classical imaging techniques like X-ray crystallography and Nuclear Magnetic Resonance (NMR) with the emerging technique of single-particle electron cryomicroscopy has become a tremendously helpful tool to understand the three-dimensional structure at near atomic-level resolution of newly discovered adhesins and their complexes. A detailed study of the structure of these molecules, both isolated and expressed on bacterial surface is a fundamental requirement for understanding the adhesion mechanism to host cells. This chapter will focus on the structure determination of such surface-exposed protein structures in both Gram-negative and Gram-positive bacterial adhesins.