A general approach to time domain digital filtering is described, and examples of some filters used in EEGIERP research are presented. Simulations are reported that evaluate the impact of the relative length of the filter weight series and the signal cycle to be flltered, the span and real-time density of the filter weights, and slow drift across the epoch being flltered. Results indicate that some fllters commonly used in the EEGIERP literature are inadequate. Frequency domain digital filtering is also briefly discussed. The fast Hartley transform, a fast but relatively unknown computational method for frequency domain filtering of ERPIEEG data, is introduced and compared with time domain flltering. Some practical recommendations are provided.The analysis of encephalographic (EEG) data, either ongoing activity or event-related potentials (ERPs), generally requires the extraction of a signal of interest from a noisy background. This filtering process may be carried out in a variety of different ways. The term digital filter refers to a wide range oftechniques that have in common the fact that they are mathematical procedures applied to discrete, numeric representations of continuous waveforms to emphasize or attenuate certain frequencies.Digitally filtering an EEG waveform in the time domain' typically involves cross-multiplying each unfiltered data point and its neighbors with a set ofweights. In effect, the weights represent a copy of the signal pattern of interest. The cross-multiplication process is repeated for each point to be filtered. The sums of these cross-products, arranged as a series, constitute the filtered waveform. An intuitive appreciation of how such a procedure can accomplish frequency-specific filtering can be gained by considering a set of weights with magnitudes forming a sine wave of a particular frequency. When data points are cross-multiplied with these weights, the sum of the crossproducts will be largest when the data predominantly consist ofa sine wave ofthe same frequency and are in phasewith the weights, such that the two sets of values rise and Portions of the introductory material in this paper are based on a more general tutorial on filtering (Cook & Miller, 1992 Copyright 1998 Psychonomic Society, Inc. 54 fall in synchrony. The sum of the cross-products will be most negative when the data have the same frequency but are inverted (180 0 out of phase). Any other pattern in the data will produce a sum closer to zero. In effect, the filter is "tuned" to detect a sinusoidal signal of a certain frequency, and signals that are nonsinusoidal or do not match the filter's frequency will not produce as much output.Digital filters ofthis type are pervasive in psychophysiology. In designing an appropriate filter for a given EEGI ERP application, the investigator faces a choice among a variety of methods for generating the weights used in the cross-multiplication function and also a choice of the set of adjacent time points to which to apply the weights. These choices affect the computational...