This volume of the Encyclopaedia offers a systematic introduction and a comprehensive survey of the theory of complex spaces. It covers topics like semi-normal complex spaces, cohomology, the Levi problem, q-convexity and q-concavity. It is the first survey of this kind. The authors are internationally known outstanding experts who developed substantial parts of the field. The book contains seven chapters and an introduction written by Remmert, describing the history of the subject. The book will be very useful to graduate students and researchers in complex analysis, algebraic geometry and differential geometry. Another group of readers will consist of mathematical physicists who apply results from these fields.