This chapter presents the classification of variables in their different categories. These categories are related to the ability to observe the system, that is, being able to obtain an estimate of the state variables of the system, especially when they are not measured. The concepts of observability and redundancy and the methods to determine observable and redundant variables are reviewed first. The unifying concept of estimability is presented next. Connections to graph theory are later made. These concepts from graph theory will be used in future chapters in design procedures. The extension of all these concepts to nonlinear systems is then covered in the last part of the chapter.