Colloidal particles stabilize emulsions both by providing steric hindrance to drop-drop coalescence and by modifying the rheological properties of the interfacial region. The effectiveness of colloidal particles in stabilizing emulsions depends in part on the ability of these particles to reside at the oil-water interface in a state of mechanical equilibrium. We present a constitutive equation to describe the rheological properties of fluid-fluid interfaces containing colloidal particles. It is shown that in addition to providing steric hindrance to drop-drop coalescence, the adsorption of particles at the oil-water interface also affects the rheological properties of the interfacial region. At sufficiently high concentration of particles, the colloid laden interface will exhibit viscoelastic behavior. Viscoelastic interfaces affect emulsion stability by retarding the rate of film drainage between coalescing emulsion droplets and by increasing the energy required to displace particles from teh contact region between droplets (i.e., by increasing the magnitude of steric hindrance).