Recent developments in geometric measure theory and harmonic analysis have led to new and deep results concerning the regularity of the support of measures which behave "asymptotically" (for balls of small radius) as the Euclidean volume. A striking feature of these results is that they actually characterize flatness of the support in terms of the asymptotic behavior of the measure. Such characterizations have led to important new progress in the study of harmonic measure for nonsmooth domains. This volume provides an uptodate overview and an introduction to the research literature in this area. The presentation follows a series of five lectures given by Carlos Kenig at the 2000 Arkansas Spring Lecture Series at the University of Arkansas. The original lectures have been expanded and updated to reflect the rapid progress in this field. A chapter on the planar case has been added to provide a historical perspective. Additional background has been included to make the material accessible to advanced graduate students and researchers in harmonic analysis and geometric measure theory. Readership Graduate students and research mathematicians interested in analysis. Reviews "This book is a good introduction to an exciting new research area on the interface of harmonic analysis and geometric measure theory. The book is very well written, with clear explanations and useful pictures. It negotiates a fine compromise between brevity and detail as it presents a subject that is necessarily somewhat technical."  Bulletin of the American Mathematical Society Table of Contents  Motivation and statement of the main results
 The relation between potential theory and geometry for planar domains
 Preliminary results in potential theory
 Reifenberg flat and chord arc domains
 Further results on Reifenberg flat and chord arc domains
 From the geometry of a domain to its potential theory
 From potential theory to the geometry of a domain
 Higher codimension and further regularity results
 Bibliography
