Continuous glucose monitoring (CGM) may include applying a periodic excitation signal via an electrode of a CGM sensor to human interstitial fluid to drive an oxidation/reduction reaction, and measuring the current through the electrode. In some embodiments, the measured current is sampled and digitized, and various harmonics of the excitation signal's fundamental frequency are extracted. A set of relationships of at least two harmonics each is generated from the spectral amplitudes of a set of pairs, triplets, etc., of the harmonics, and the set of relationships is mapped to a glucose concentration such as based on the contents of a harmonic relationship database having a pre-existing set of harmonic relationships and glucose concentrations to which those sets of harmonic relationships correspond, for example. Numerous other embodiments are provided.