This monograph illustrates how elementary harmonic measure arguments have broad applications. The author presents some recent results on harmonic measure and applications of harmonic measure estimates to problems in analysis and spectral theory. Most of the results included are not available in any other book. The treatment is elementary in that Brownian motion is not used--the introduction gives all the background needed for following the text. Chapters cover length sums, level curves of conformal mappings, interpolating sequences, nontangential limit sets, Makarov's theorems, and periodic spectra of Hill's equation.