Experimental and theoretical work defining the effects of Brownian motion and dispersion, steric, and electrostatic forces on the rheology of suspensions is reviewed. Data from well‐characterized monodisperse systems are interpreted through analyses, primarily of pair interactions, which relate the bulk stress to the suspension microstructure. The magnitudes of the interparticle forces relative to Brownian motion determine the nature of the microstructure at rest: neutrally stable, stable due to strong electrostatic or steric repulsions, or flocculated by strong attractive forces. The rheological behavior varies correspondingly from moderately shear thinning with Newtonian low‐ and high‐shear viscosities for hard sphere interactions to solidlike in the low‐shear limit when multiparticle attractive or repulsive forces dominate Brownian motion.