Abstract Crystalline basement rocks in Thailand are generally overlain by Cambrian sedimentary rocks, locally containing Cambrian fossils. Consequently their age was regarded as Precambrian. Through the application of isotope geochronology, it is now however evident that this was an over-simplification. The definition of a ‘basement rock’ used in this chapter follows that of Landes et al . (1960 ) and is considered to be any metamorphic or igneous rock (regardless of its age) which is overlain unconformably by a sedimentary sequence. The basement rocks are mostly regionally metamorphosed rocks which are easily distinguished from low-grade sedimentary rock types in Thailand. The typical grade of meta-morphism is generally between medium and high; segregation banding and even migmatization is observed. The Department of Mineral Resources (2001) and Salyapongse (2002 ) give a detailed petrographic description of the main occurrences. The occurrence of gneisses in Thailand was described for the first time by Heim & Hirschi (1939 ). Brown et al . (1951) and Buravas (1952 ) assigned a Precambrian age to all gneisses and weakly metamorphosed rocks in Western Thailand. Javanaphet (1969) also assumed a Precambrian age for the gneisses, while less metamorphosed units were classified as Palaeozoic. A summary of the distribution of gneisses investigated is shown in Figure 2.1 . Baum et al . (1970 ) described for the first time a detailed stratigraphy of the metamorphic rocks in Northern Thailand and assigned Precambrian ages to the gneisses. However, according to those authors the gneisses were overprinted by granitization