This monograph presents a systematic analysis of diagrams as visual representations of factual knowledge. The analysis shows that the design process may be divided into three phases: a data classification phase, a graphical decision phase, and a layout phase. Performed in this order, the three phases more or less reflect the design process of a human expert. They also serve as a basis for a constructive theory for diagram design, which is the main focus of this book. Another interesting topic is only touched upon, namely the commonalities of automatic graphics and text design.