Radioactively contaminated land and regions of high natural background radioactivity are causes of concern in several parts of the world. Anthropogenic sources are associated with large-scale, historic uranium mining/processing, with facilities that operated during the earliest years of nuclear power and nuclear weapons programs, and with accidental releases of radioactivity (e.g., Chernobyl, in 1986). Since 1990, much information has emerged on anthropogenic sources, especially from eastern European and former Soviet Union countries. Multinational projects such as the Swiss National Science Foundation Scopes 2000 (scientific cooperation between eastern Europe and Switzerland) Russia-Ukraine-Switzerland project have assessed specific problem sites. Remediation projects and contaminated sites are being used as case studies to untangle geochemical and hydrogeological processes controlling radionuclide behavior in groundwater systems.