Introduction Adhesion is essential in biology. Intercellular interactions maintain the structural hierarchy of all multicellular organisms across all anatomical length scales. Cells transduce mechanical signals and respond by regulating adhesion, motility, and differentiation. Other adhesive interactions are central to immunity. Pathogenic microorganisms use adhesive interactions with cells in the first steps in infection. Determining the molecular mechanisms underlying these processes is central to understanding the fundamental basis of related diseases and to developing strategies to treating or preventing disease. Force probe techniques are ideal tools for investigating the mechanisms that control the biological adhesion and the strength of biomolecular linkages. Although the adhesion strength is the central parameter defining function in many biological interactions, adhe- sion measurements alone are insufficient to determine the relationships between molecular architectures and their mechanical function. In particular, both the range and magnitude of forces are often functionally relevant, particularly in complex environments such as the cell surface. Many adhesion proteins as well as other glycoproteins on cell surfaces are large, and extend several tens of nanometers from the cell membrane. The range of protein interactions, facilitated by these large structures, is often thought to be critical to their function. In addition, while single-molecule measurements reveal a wealth of information regard- ing individual molecular linkages, the collective behavior of tens to thousands of proteins or macromolecules typically determines cell-cell or cell matrix adhesive strengths. It is therefore important not only to explore the mechanical strengths of single molecular bonds, but also to determine how populations of bonds govern biological interactions. The surface force apparatus SFA is uniquely suited to investigations of molecular adhesion because it quantifies both the molecular forces between surfaces and the distances over which the forces act. It also quantifies the adhesion between extended surfaces such as membranes, which are arguably more relevant to cell interactions. The SFA is distinct among force meas- urement techniques due to its ability to determine absolute intersurface separations to within 0.1 nm, while quantifying weak, noncovalent interactions with bond energies on the order of the thermal energy k B T (2.48 kJ/mole or 0.59 kcal/mole at room temperature). This chapter describes the surface force apparatus technique and several of its capabilities. I also describe key examples of how data obtained from SFA measurements generated unique information regarding the relationships between the structures and mechanical functions of proteins that mediate cell adhesion in immunity and the nervous system. These examples focus Surface Force Apparatus Measurements of Molecular Forces in Biological Adhesion Deborah Leckband 1