This book is concerned with conceptual modeling, design and specification of in- formation systems. Conventional information systems design starts with separating data from op- erations, designing each with its own collection of concepts, methods, tools - and people: conceptual modeling for the imformation structure, and program design for the application programs. The separation carries through until the final implemen- tation: data are collected in databases and managed with database management systems, and application programs are implemented with programming languages. This approach tends to suffer from a problem known as impedance mismatch. The basic paradigms underlying databases and programs - modeling, design, languages, and systems - do not fit easily together: there are incompatible type systems, data formats, operation modes, etc. The object-oriented paradigm promises to overcome these problems: a system is viewed as a community of interacting objects, each incorporating data and oper- ations. While we still have object-oriented programming languages incompatible with object-oriented database systems, ideas and approaches seem to converge to- wards homogeneous software systems dealing with both data and operations in a uniform way. Viewing a system as a community of interacting objects does not solve all prob- lems. Beyond the object concept, abstraction and structuring principles are needed, together with languages and methods to work with them.