Cancer immunotherapy is fast emerging as a promising treatment approach for many cancers. However, outcomes remain highly variable across individuals, as only a minority of patients respond, and addressing this variability is one of the most active areas of immunotherapy research. Studies have shown that the infiltration of tumors by immune cell subtypes is one of the most significant prognostic indicators of disease-free survival across a large number of cancers. However, we remain limited in our ability to non-invasively sample the continuously evolving tumor micro-environment for the presence of immune cells that can give us critical insights into the response status of the patient. Rare-earth nanoparticles are an exciting new class of optical materials very attractive for high resolution imaging of deep tissue structures. They have narrow and tunable emission bands in regions of near-zero tissue autofluorescence (1300-1700nm), large anti- Stokes shift enabling clear separation of excitation and emission wavelengths, high photostability for continuous and repetitive imaging, and very low intrinsic toxicity in vivo due to their exceptional chemical inertness, making them attractive for clinical translation. Moreover, near-infrared optical imaging technology offers several advantages over conventional clinical imaging tools in terms of resolution, cost, safety, and repeatability. In this thesis, we show that near-infrared imaging using rare-earth nanoparticles can serve as a powerful tool to non-invasively image the distribution of immune cells in a tumor. We first present nanoparticle synthesis strategies for the generation of small but ultra-bright rare earth nanocrystals necessary for deep tissue imaging of rare cell types. We then evaluate a variety of surface modification approaches and present new methods for facile and reproducible phase-transfer of these nanoparticles to enable further biofunctionalization and cell targeting. Finally, we demonstrate that these nanoparticles can be used for high resolution in vivo imaging of tumor-infiltrating T-cells in a melanoma tumor model. The ability to noninvasively monitor the immune contexture of a tumor during immunotherapy will yield valuable real time insights into the response status of a patient, and could lead to early identification of responding and non-responding patient cohorts.Ph.D.