INTRODUCTIONImaging the single-cell dynamics of the immune system within an intact environment requires the ability to look deep inside tissues and organisms with spatial and temporal resolutions adequate to track cell morphology, motility, and signaling processes, all while minimizing perturbation of the system under study. Fluorescence techniques are highly suited for this purpose, permitting both labeling of specific cells, organelles, or proteins and functional readout of physiological events, and two-photon microscopy allows these processes to be visualized within native tissue environments. Adoptive transfer, as described here, is the generally preferred method for introducing labeled cells of interest into a host animal for immunoimaging. Cells are derived from a donor animal with a genetic background identical to that of the host and can either be endogenously fluorescent (e.g., isolated from a transgenic mouse expressing the fluorescent protein) or can be labeled before transfer. Typically, transferring 2-6 × 106 labeled cells of a given type results in an appropriate cell density for two-photon imaging.