  The connection between friction and jamming in granular media, molecular
glasses, and complex fluids is explored. The paper first reviews the way
friction is measured, the types of results that are observed, and what is known
about the geometry of contacts between macroscopic surfaces. Then simple models
for the origin of static friction are described. These are unable to explain
the universal observation of static friction between macroscopic objects. The
effects of surface roughness and chemical heterogeneity are discussed, and
shown to yield exponentially weak static friction in our three dimensional
world. In contrast, jamming of the "third bodies" that are present between most
surfaces is shown to produce static friction that is consistent with
macroscopic measurements. Experimental and simulation studies of jamming of
molecularly thin fluid films are described and compared to studies of bulk
glass transitions. The paper concludes by considering the unsteady stick-slip
motion that often arises when systems become unjammed by sufficiently large
stress. Different types of stick-slip motion are identified, and some of its
origins are explored.
