In situ treatment of contaminants by the direct injection of a reagent solution into the subsurface is an emerging remedial strategy. Three case studies are presented where tracers were used to evaluate various design and operational components at sites treating contaminants in situ. Tracers were used to determine site-specific design parameters including groundwater velocity, effective radius of injected fluids, and required frequency of injections. Additionally, tracers were used to verify hydraulic connectivity between the injection field and down-gradient observation wells, provide information on hydrogeology and solute-transport behavior (i.e., permeability differences, preferential channeling, and fractured flow) within the targeted treatment areas, and in two instances, used in conjunction with reagent delivery for determination of decay half-life coefficients. These case studies demonstrate that tracers can be used as practical tools to aid in designing full-scale systems and evaluating operation and long-term performance of in situ treatment strategies.