When considering the reconstruction of a signal, you should already be familiar with the idea of the Nyquist rate. This concept allows us to find the sampling rate that will provide for perfect reconstruction of our signal. If we sample at too low of a rate (below the Nyquist rate), then problems will arise that will make perfect reconstruction impossible - this problem is known as aliasing. Aliasing occurs when there is an overlap in the shifted, perioidic copies of our original signal's FT, i.e. spectrum.

In the frequency domain, one will notice that part of the signal will overlap with the periodic signals next to it. In this overlap the values of the frequency will be added together and the shape of the signals spectrum will be unwantingly altered. This overlapping, or aliasing, makes it impossible to correctly determine the correct strength of that frequency. Figure 1 provides a visual example of this phenomenon: