The fate and mobility of dissolved (aqueous phase) and nonaqueous phase contaminants in groundwater are described. The different transport processes and their respective conceptual models are explained. The related equations describing their effects in heterogeneous geological media are derived. In situ remediation and natural attenuation are introduced briefly. Site characterization methods and assessment of transport times using artificial tracers as well as isotopes and natural environmental tracers are outlined. Groundwater ages and residence times are discussed in different contexts. Process-based groundwater vulnerability and protection concepts are presented.