This book represents an expanded account of lectures delivered at the NSF-CBMS Regional Conference on Singular Integral Operators, held at the University of Montana in the summer of 1989. The lectures are concerned principally with developments in the subject related to the Cauchy integral on Lipschitz curves and the T(1) theorem. The emphasis is on real-variable techniques, with a discussion of analytic capacity in one complex variable included as an application. The author has presented here a synthesized exposition of a body of results and techniques. Much of the book is introductory in character and intended to be accessible to the nonexpert, but a variety of readers should find the book useful.