Understanding the complex nature of biochemical processes in living cells is a fundamental pursuit of cell biologists and biochemists. The ability to probe these processes with fine control is of interest to DoD as it is integral for the development of novel therapeutics to protect the warfighter against new and emerging threats. Our laboratory has developed high quality nanomaterials with unique size-dependent photo-physical properties that make them superior to traditional fluorescent materials (organic dyes, fluorescent proteins) that have heretofore been used to probe these processes. These materials, known as semiconductor nanocrystals or quantum dots (QDs), exhibit 1-extreme brightness and resistance to photo- and chemical degradation, 2-efficient excitation and visualization within cellular environments, 3-broad excitation spectra coupled with narrow, symmetric, size-dependent emission, 4-efficiency as fluorescence resonance energy transfer (FRET) donors to proximal FRET acceptors and 5-two-photon action cross-sections that range up to almost two orders of magnitude larger than organic dye molecules (Fig. 1a). Currently our efforts are focused on understanding how to control these unique nanomaterials with cellular environments to not only harness their ability to serve as extremely sensitive probes for labeling/imaging of cellular structures (Fig. 1b) but to also realize their potential as materials that can be used to control and modulate cellular behavior.