Although humans live in a world dictated by the steady march of time, and that we typically experience events in the time domain, there is an alternate realm in which periodic time series can be analyzed. Apart from the mean and trend, more complex and periodic characteristics of a time series may be revealed by analyzing the frequency content of the data. For ocean sciences, this type of analysis can be divided into two branches: spectral analysis and harmonic analysis. Both have the intent to extract information about the magnitude and relations of the periodicity, or repeated nature of signals in the time series. Recently, more advanced techniques, such as wavelet analysis, have aided some investigations, but they are beyond the scope of this brief explanation.