The general features of a computer-controlled x-ray diffractometer system are described. A standard four-axis Geiger counter single-crystal diffraction configuration is made automatic by attaching encoders to all shafts with provision made for electronic input of all settings and readout of all data. A small solid-state computer is made an integral part of the system, which permits the system to be used in a decision-making closed-loop manner. The mode of operation can be changed by changing the program in the computer. © 1963 The American Institute of Physics.