Monitoring the behavior of single molecules in living cells is a powerful approach to investigate the details of cellular processes. Owing to their optical, chemical and biofunctional properties, semiconductor quantum dot (QD) probes promise to be tools of choice in this endeavor. Here we review recent advances that allow ever more controlled experiments at the single-nanoparticle level in live cells. Several examples, related to membrane dynamics, cell signaling or intracellular transport, illustrate how single QD tracking can be readily used to decipher complex biological processes and address key concepts that underlie cellular organization and dynamics.