Picosecond fluorescence depolarization measurements of electronic excitation transfer (EET) are used to probe the structures and dynamics of polymer blends. Fluorescent chromophores are covalently incorporated into one of the polymers in the blend, and the rate of fluorescence anisotropy decay is measured using time correlated single photon counting. Analysis of the fluorescence anisotropy decays yields the time-dependence of the EET. EET is a very sensitive probe of interchromophore distance; therefore it can be used to examine the single chain structure and the spatial distribution of chains in the blend. At temperatures above the phase separation temperature, macroscopic phase separation eventually occurs. However, prior to macroscopic phase separation, the structure and dynamics of the onset of phase separation is not understood. Using EET, we are able to observe the aggregation of as few as two polymer chains at the onset of phase separation. These small aggregates are referred to an nanodomains. By lowering the sample temperature below the blend's glass transition temperature, the nanodomains can be trapped in the solid material. The number of chains in the nanodomains and the growth of the nanodomains are determined by analyzing the EET data with a detailed theory that relates the spatial distribution of chromophores to the EET observables. Varying the rate of heating allows us to study the kinetics of formation of these nanoscopic heterogeneities, and to observe the phase separation process at the molecular level, from its onset through macroscopic phase separation.