Tracking single molecules inside cells reveals the dynamics of biological processes, including receptor trafficking, signaling and cargo transport. However, individual molecules often cannot be resolved inside cells due to their high density in the cellular environment, plus it is difficult to see spatial and temporal features, such as signal transduction events at the cell surface or on intracellular compartments, with single molecule resolution. To address these problems, researchers at the University of California, Berkeley, have developed the PhotoGate device and methods in order to control the number of fluorescent particles in a region of interest. By deploying PhotoGate and applying patterned photobleaching, they have demonstrated the tracking of single particles at surface densities two orders of magnitude higher than the single-molecule detection limit. Additional experimentation enabled the observation of ligand-induced dimerization of epidermal growth factor receptors on a live cell membrane, and also measurements of the binding and the dissociation rate of single adaptor protein from early endosomes in the crowded environment of the cytoplasm. The innovative approach enables tracking of single particles at high spatial and temporal resolution, and for mapping of molecular trajectories, as well as determining complex stoichiometry and dynamics, and drives the art towards video-rate imaging of live cells with molecular (1–5 nm) resolution.