I N THE HIGH-ENERGY physics laboratory the most remarkable development that has occurred in the last five years has been the introduction of the digital computer as an active part of the experimental apparatus. This development has been due chiefly to the increased number and complexity of detection elements in a typical high-energy experiment and the dramatic decrease in cost of computing equipment. The point here is not the increased use of a computer per se to do numerical calculations in analysis of raw experimental data but its use “on line” as an intrinsic element in a closed control loop to modify the experiment while it is in progress.