Cells sense, affect and respond to their environment through the fundamental function of adhesion. Several types of adhesion sites, which are mediated via dynamically maintained multi-protein structures, anchor extracellular-matrix proteins to the cytoskeleton. Despite considerable efforts, the long-standing questions of how adhesion sites are formed, structured and regulated remain unanswered. In this research plan we will investigate desmosomes and adherens junctions by cryo-electron tomography of cells and tissue. The principal objectives are: (a) to visualize the molecular architecture and reveal the structural differences of the adhesion sites under various conditions and influences, i.e. mutations, wounds, etc. (b) to reveal their molecular association to the cytoskeleton (intermediate and actin filaments respectively), and to chart the network of interactions underlying cellular adhesion, and (c) to develop novel pattern recognition and classification techniques in order to structurally characterize the adhesion sites in toto by cryo-electron tomography of vitreous sections. We will use pattern recognition techniques and locally averaged cryo-electron sub-tomograms to quantify the macromolecular complexes in terms of stoichiometry and protein interactions in situ at high resolution (~3 nm). In particular, we aim to reveal how a pool of constituent proteins is organized in the two adhesion sites. Significant amounts of information coming from immunogold electron microscopy, fragments from X-ray structures, force measurements with atomic force microscopy, and structural bioinformatics will be integrated into our cryo-electron tomograms. This research will pioneer structural comparisons of protein networks at nanometer resolution in situ and in toto. The experimental and theoretical methods that will be developed would be indispensable for studying any spatially constrained protein network whose state depends on local properties.