This Topic revisits measuring the size of a signal, but rather than doing this in the time-domain, it is now considered in the frequency-domain. By considering the natural size of a signal's spectrum, it is shown that energy in the time-domain is equal to the energy in the frequency-domain. This has applications for analysing signals in both domains, but also dealing with the case where the theoretical calculation of energy in the one-domain is difficult, but is simpler in the other-domain. Using this method, it is possible to create a number of useful identities, as shown in the associated self-study questions.