A brief theory of correlation, as applied to the recognition of a specific pattern in a binary signal, is presented. The digital shift register, which lends itself well to the practical application of the theory of correlation, is discussed. By digitizing the binary input signal and applying it to a closed-loop delay line, a time-compressed version of the input signal circulates on the line, and the delay line can perform the same function as the digital shift register, with possible advantages of size and cost over the shift register. Sample parameters are postulated for a shift register, and a shift register employing a magnetostrictive delay line is designed. The operation of the designed delay line shift register is then discussed in detail.