During the 1989 European ASL Summer Meeting in Berlin, the authors gave a series of eight lectures (short course) on the topic of the title. This survey article consists basically of the lecture notes for that course distributed to the participants of that conference. We have purposely tried in this printed version to preserve the informal style of the original notes.Let us say first a few things aboui the content of these lectures. Our aim has been to present some recent work in descriptive set theory and its applications to an area of harmonic analysis. Typical uses of descriptive set theory in analysis are most often through regularity properties of definable sets, like measurability, the property of Baire, capacitability, etc., which are used to show that certain problems have solutions that behave nicely. In the theory we will present, definability itself, in fact the precise analysis of the “definable complexity” of certain sets, will be the main concern. It will be through such knowledge that we will be able to infer important structural properties of various objects which will then be used to solve analysis problems.The first lecture provides a short historical introduction to the subject of uniqueness for trigonometric series, which is the area of harmonic analysis whose problems are the origin of this work. As is well known, it was Cantor who proved the first major result in this subject in 1870, and it was his subsequent work here that led him to the creation of set theory.