This book introduces and describes in detail the SEQUAL framework for understanding the quality of models and modeling languages, including the numerous specializations of the generic framework, and the various ways in which this can be used for different applications. Topics and features: contains case studies, chapter summaries, review questions, problems and exercises throughout the text, in addition to Appendices on terminology and abbreviations; presents a thorough introduction to the most important concepts in conceptual modeling, including the underlying philosophical outlook on the quality of models; describes the basic tasks and model types in information systems development and evolution, and the main methodologies for mixing different phases of information system development; provides an overview of the general mechanisms and perspectives used in conceptual modeling; predicts future trends in technological development, and discusses how the role of modeling can be envisaged in this landscape.