Communication channels are often modeled as time-invariant or time-varying linear filters. If the length of the channel impulse response is significant compared to the baud period, it will introduce amplitude and phase distortion into the transmitted waveform. The overall pulse, consisting of the concatenation of the transmitter pulse shaping filter, nonideal channel, and receiver matched filter is no longer a Nyquist pulse and intersymbol interference (ISI) will be introduced. ISI will degrade the bit error rate performance, and if the ISI is severe enough, errors will occur even in the absence of noise. Consequently, it is necessary to compensate for ISI at the receiver and this compensation is accomplished by using an equalizer. This chapter introduces the basic concepts of time-domain equalization. This chapter first considers the receiver front-end processing needed for channels with ISI. Afterwards, a simple linear symbol-by-symbol time-domain equalizer is studied called a zero-forcing equalizer. Subsequently, partial response signals are considered, where controlled ISI is introduced at the transmitter to achieve bandwidth efficient communication within the Nyquist bandwidth all the while employing realizable filters.