Geospace disturbances refer collectively to the variations of the geomagnetic field and the trapped particle populations in the near-Earth space. These are the result of transient and recurrent solar activity, which consequently drives the variable solar wind. They may appear in multiple timescales, from sub-seconds to days, months and years. Wavelet analysis is one of the most popular, and powerful, methods in the study of these variations, as it allows for the local decomposition of non-stationary time series in frequency (or time-scale) and time simultaneously. This article is a review of the wavelet methods used in the investigation of geomagnetic field oscillations, which underlines their advantages as spectral analysis methods and demonstrates their utilization in the interdependence of multiple time-series. Lastly, the proper methodology for the accurate estimation of the power inferred from geophysical signals, applicable in quantitative studies, is included and is publicly available at the database of the University of Athens.