Droplets coated with a polymerized interface formed by interfacial cross-linking polymerization are a good example of capsules. Capsules are also thought to be models of red blood cells (RBCs). The rheological behavior of suspensions of capsules differs from that of suspensions of solid particles or liquid droplets. At low shear rates the capsules behave as rigid particles and rotate as a rigid body independently of the viscosity of the enclosed liquid. The tank-treading motion of capsules has been observed experimentally in the case of RBCs. The rheology of suspensions of deformable particles is intimately linked to the deformation and orientation of particles in the shear field. Knowledge of the motion and deformation of a single representative particle in a viscous shear flow is required to calculate the dipole strength term contributing to the bulk stress of the suspension. When the system is subjected to a shear flow, the capsule deforms under the influence of the viscous stresses.