Applications of quasi-elastic light scattering spectroscopy (QELSS) to the study of protein and polymer solutions are discussed. In binary systems, quasielastic light scattering measures the mutual diffusion coefficient Dm of the macromolecules. For spherical macromolecules, Dm and its concentration dependence can be quantitatively interpreted in terms of hydrodynamic and direct interactions between the diffusing particles in solution. Also discussed are optical probe experiments, in which QELSS spectra of ternary probe: matrix polymer: solvent systems are obtained. The diffusion of dilute probes through a complex probe:matrix system reveals detailed information about the dynamics of the matrix. A direct extension of the orthodox theory for interacting, diffusing spheres successfully describes the dynamics of random-coil polymers in solution. Probe diffusion and other measurements of polymer transport coefficients suffice to reject some alternative models of polymer dynamics.