Abstract: The spectrum of radiated noise from ships consists of a continuous broadband spectrum and a discrete narrow-band line-spectrum. A crucial issue in a passive sonar system is detection of the spectral lines. A review of techniques for detection of spectral lines is given in this paper. Recent developments in this field show that, in order to detect spectral lines at a very low signal to noise ratio, combination of more than one method is usually adopted, and multiple frame spectra are used.