Ten years have passed since the last symposium on "Rock Deformation" was held in Los Angeles. The intervening period has seen striking advances in X-ray and experimental structural petrology. The Symposium of the Working Group on X-Ray and Experimental Structural Petrology, held at the invitation of the Deutsche Forschungsgemeinschaft on 17 and 18 February, 1969, in the Mineralogy Institute of the Technische Hochschule Darmstadt, was intended to display the progress made in this field. A meeting on the same had been held in 1968 at the Mineralogy Institute of the Technische Hochschule Aachen. The outcome of many years of effort has been the development of new, automated in­ struments for the X-ray processing of specimens. The X-ray and optical data so obtained are processed by computer to give complete texture diagrams; it is also possible to rotate the coordinates, and to make an indirect determination of data which have not been measured. In experimental structural petrology, the temperature and pressure dependence of the translational system has been studied in minerals from various types of rock formation which had not previously been investigated, and deductions were made concerning certain mineral parageneses. The original contributions on field findings were conceived and executed so as to point up the possible links with experimental work on structural petrology. They determine for a given mineral facies the pressure and temperature ranges to be applied in experimen­ tal studies of deformation in single crystals and in rocks.