This book contains the proceedings of the International Workshop on Operator Theory and Applications held in Faro, Portugal, September 12 to 15, 2000. It includes 20 selected articles centered on the analysis of various classes of singular operators, the factorization of operator and matrix functions, algebraic methods in approximation theory, and applications in diffraction theory. Some papers are related to topics from fractional calculus, complex analysis, operator algebras, and partial differential equations.