Waste products have been commonly characterized by leaching tests and technical investigations with respect to applications. Many of these investigations are not related to the composition of waste or only consider the element composition. Few research has been conducted on the leaching behavior with respect to the chemical speciation and the mineral phases. The petrology is concerned with the composition of geogenous and synthetic rocks in relation to their origin and their occurrence. The quality of solid waste treatment products can be related to their origin with respect to educt qualities and the conditions during treatment. The knowledge of the origin in comparison to geogenous sediments or rocks is a key to predict the long-term behavior. The more precise the knowledge of the origin or genesis of waste product qualities the better the conditions for optimizing a treatment system with respect to potential hazards end resource capacities. Thus the petrological approach in the context of waste products is thought as a link between geochemical investigations and waste treatrnent technologies. The approach is pointed out by the example of municipal solid waste incinerator bottom ashes of grate cystems.