Backward-in-time modeling of groundwater contamination simulates the movement of contaminants away from their receptors and toward their possible sources. This modeling technique is useful if data is available about the current location of contamination, and information is needed about the contaminant's past, such as the source location or the release time of the contaminant from a known source. Backwardin- time modeling solves the adjoint of a forward contaminant transport equation such as the advectiondispersion equation. The form of the adjoint is very similar to the form of the forward equation, and therefore backward-in-time modeling can be done using any numerical code that solves the forward contaminant transport equation. Applications include source identification, capture zone delineation, groundwater vulnerability, groundwater age dating, and others. Examples will be presented of applications of the method using MODFLOW and MT3D.