The non-invasive imaging of immune cells has become an increasingly relevant approach for understanding immune cell dynamics within a patient in real time, especially within the context of cancer immunotherapies. Immune checkpoint inhibitors (ICIs), for example, have had unprecedented success in treating cancers that were once refractory to previous lines of treatment. Despite this success, only a small fraction (~12%) of treated patients respond favorably to treatment, and immune related adverse events, that can be life threatening, occur in a significant percentage of those treated. Within the past 5 years, researchers in the field of molecular imaging have identified more direct and promising ways of monitoring and assessing a patient’s response to immune checkpoint inhibitors, which has included imaging immune cell phenotypic markers, like CD8, or immune cell activation markers, like ICOS or CD69, using immunoPET. In parallel to the development of ICIs, adoptive cell therapies, such as chimeric antigen receptor (CAR) T cell therapies, have demonstrated remarkable success in treating B-cell hematological malignancies that were previously refractory to treatment. However, much can still be learned about CAR T cell dynamics within the patient, and the optimization of CAR T cell efficacy and persistence for treating solid tumors is still underway. Additional challenges include, adverse events, like cytokine release syndrome and immune effector cell-associated neurotoxicity syndrome, which can be life threatening if not managed early and effectively. Therefore, imaging approaches that allow for the quantitative monitoring of CAR T cell trafficking and activation would be highly beneficial. To address this need, constitutive PET reporter gene and PET reporter probe pairs have been developed and have showed promise for CAR T cell monitoring in vivo. In this thesis, I have detailed the development of PET and optical imaging probes for assessing immune cell activation, in vivo. First, I describe the development of two immunoPET probes for the preclinical monitoring of response to immune checkpoint inhibitors: I) a Zr-89-radiolabeled anti- CD69 full length antibody, and II) a Ga-68-radiolabeled anti-CD69 cys-diabody. Secondly, I describe the in vitro development and optimization of NFAT/AP-1- inducible dscGFP.fLuc and eDHFR.dscGFP(df) constructs for bioluminescence imaging (BLI) and PET monitoring of CAR T cell trafficking in vivo. Together these approaches show promise for monitoring the activated state of native and engineered T cells in vivo.