Single particle tracking (SPT) provides the necessary spatial and temporal resolution to acquire information on the diffusional and interaction dynamics of individual proteins. Quantum Dots (QDs) are bright and photostable probes that make long-term SPT possible and their large Stokes shifts facilitate multi-color SPT through single wavelength excitation. We developed two-color single QD tracking methods and analyses to characterize receptor dynamics as well as transient protein-protein interactions down to the ∼10 nm scale. The analyses of receptor trajectories provide quantification of dynamic parameters that cannot be obtain from traditional biochemical techniques. Specifically, we developed a Hidden Markov Model (HMM) approach to identify and extract the lifetime and kinetic rates for dimerization and map regions of the plasma membrane explored by monomeric and dimeric receptors. We apply this technique to study the interplay between transmembrane receptor tyrosine kinases, erbB1 and erbB2, using QD-conjugated ligands, variable fragments of heavy-chain only antibodies (Nanobodies), and reduced monoclonal antibody fragments. Single-color QD-SPT is used to characterize the receptor diffusional dynamics in the presence or absence of ligands or inhibitors. The diffusion coefficients we measured by single QD tracking are consistent with published values obtained from tracking probes labeled with organic fluorophores. Using spectrally distinct QDs and simultaneous two-color imaging, we directly observe transient homo- and hetero-dimers even in the absence of ligand. Addition of EGF, the ligand for erbB1, prolonged dimer interactions. Current efforts seek to extract dimerization parameters from these two-color data sets using the HMM analysis.