Many different filters exist to remove unwanted signals from time series of ocean currents and sea levels, and in this paper we examine two: a tidal harmonic ''filter'' and a discrete Fourier transform (DFT) filter. DFT filters are particularly easy to use if working in the frequency domain, but as with all filters is is necessary to understand the effects of a DFT filter before it can be used with complete confidence. A number of sample time series are used, some artificial and some real, to test DFT filters. It is shown that ''ringing'' in the retransformed time series can be minimized by careful choice of the filter bandwidth and the amount of tapering of the sides of the filter. ¿ American Geophysical Union 1988