The ability to analyze the frequency domain with the Fast Fourier Transform (FFT) has opened up a whole new set of opportunities in digital oscilloscopes. However, the user needs to understand the capabilities and constraints of both the measurement techniques and the instruments that perform them. With the aid of practical examples, this paper helps the user understand the theory behind the techniques, to identify the measurements that the FFT can perform, and to set up the oscilloscope for optimum results.