Localization of targets imbedded in a heterogeneous background medium is a common problem in seismic, ultrasonic, and electromagnetic imaging problems. The best imaging techniques make direct use of the eigenfunctions and eigenvalues of the array response matrix, as recent work on time-reversal acoustics has shown. Of the various imaging functionals studied, one that is representative of a preferred class is a time-domain gener-alization of MUSIC (MUltiple SIgnal Classification), which is a well-known linear sub-space method normally applied only in the frequency domain. Since statistical stability is not characteristic of the frequency domain, a transform back to the time domain af-ter first diagonalizing the array data in the frequency domain takes optimum advantage of both the time-domain stability and the frequency-domain orthogonality of the relevant eigenfunctions.