Modern communications theory and information theory have placed considerable emphasis upon the problem of detection and extraction of signals in noise backgrounds. Some of the methods and equipment developed have possible geophysical applications. This paper presents a discussion of the delay line method of filter approximation. Magnetic recording delay line equipment which permits approximation of a wide variety of linear filtering and correlation operations is described. The fundamental theory of linear operator filtering by delay line techniques is outlined and some possible applications to processing seismic data are briefly discussed.