In the course of industrialization in the 19th century, manufactured gas plant sites were built in almost every larger town in Europe and the United States. Organic and inorganic contaminations pose a continuing threat to groundwater resources at these locations even though the operations were terminated decades ago. The different coal tars and tar oils present in the subsurface are composed of a huge variety of organic compounds often possessing a high toxic potential, such as monocyclic, polycyclic, and heterocyclic aromatic hydrocarbons. In addition to these inorganic contaminants, cyanides and heavy metals can be found due to dumping of by-products and wastes. A multitude of factors influence the fate of these compounds and an increasing number of studies deal with this subject. However, key processes are still not well understood. One example is the mobilization of colloid associated contaminants by extreme single events, which can be responsible for the bulk contaminant load in the seepage water. Another example is the role of microbial populations and communities in cooperative degradation pathways of organic pollutants. We aim for a comprehensive overview on the present knowledge about the processes, which determine the fate of organic and inorganic contaminant at such sites. This way, a sound basis is provided for the identification of future research directions, evaluation of risk, and development of remediation techniques.