This textbook is about quasi-realistic models in atmospheric and oceanic sciences. Understanding the basis and limitations of these models is important since far reaching decisions about the environment are based on these models. It is novel in that it goes beyond a technical discussion of these quasi-realistic models and emphasizes their role and utility in generating new useful knowledge about the system. The book is written in a generally understandable way, with technical details relegated to a set of comprehensive appendices. The line of reasoning is illustrated by numerous examples, from both applied and fundamental research. It is a source of information for graduate students and scientists alike working in the field of environmental sciences.