Oxidative stress, or the imbalance of reactive oxidative species and antioxidants, is implicated in a wide variety of physiological functions and diseases. Currently, little is known about the biological concentrations and the exact roles of individual species. In particular, the cellular concentration of hydroxyl radical and the etiology of this reactive oxygen species in disease states are unclear. The photophysical properties of luminescent lanthanide-based imaging agents and the magnetic properties of fluorinated contrast agents make them favorable candidates to monitor oxidative species in biological environments. Luminescent lanthanide-based probes for hydroxyl radical are presented. These probes utilize aromatic acid pre-antennas that sensitize terbium emission upon hydroxylation. The ability of hydroxylated and non-hydroxylated aromatic acids including benzoate, benzamide, isophthalate, isophthalamide, trimesate, and trimesamide to sensitize Tb DO3A was evaluated by time-delayed luminescence spectroscopy. The formation of a weak ternary complex between hydroxytrimeasamide and Tb-DO3A was confirmed by temperature-dependent titrations. The luminescence response of the bimolecular Tb DO3A and trimesamide probe to hydroxyl radical generated by the photolysis of hydrogen peroxide was investigated. The system exhibits excellent selectivity for hydroxyl radical over other biologically relevant reactive oxygen and nitrogen species. Next, fluorinated magnetic resonance imaging contrast agents responsive to hydroxyl radical are described. The 3,5-difluorobenzoic acid probe is water soluble and ratiometrically responds to hydroxyl radical. Upon hydroxylation, a fluoride ion is released. The relative signal intensity of the product and that of the unreacted contrast agent can then be used to monitor the analyte in a ratiometric manner by 19F NMR and 19F MRI. The selectivity of the system towards hydroxyl radical compared to other reactive oxygen and nitrogen species is also measured. Paramagnetic, lanthanide-based contrast agents incorporating the sensing moiety are also evaluated for increased sensitivity of detection compared to the diamagnetic analogs. Additionally, a family of lanthanide-based luminescent complexes based on a macrocyclic core featuring different sensitizing antennas and variable pendant arms are investigated in terms of their biological compatibility. The cellular uptake of Tb-DOTA complexes containing hydroxyisophthalamide (IAM), methoxyisophthalamide (IAM(OMe)), or phenathridine (Phen) antenna were comparable despite their differences in hydrophobicity. The luminescence quenching of Tb-DOTA-IAM(OMe) was also investigated in cell lysate by time-delayed spectroscopy. Pendant arms varying in hydrophobicity and charge were used to evaluate the effect of structural and electronic properties on cellular viability and cell association as measured by a MTT assay and ICP-MS, respectively. Regardless of the amide substituents, complexes based on Tb-DOTAm-IAM(OMe) core exhibited low cytotoxicity and low cellular association. Thus, complexes based on this platform are well-suited for the detection of extracellular analytes.