Book excerpt: Mullite is the most important crystalline phase in fired products belonging to the Al2O3–SiO2 mullite system, such as whiteware articles used in daily life (e.g., low- and high-temperature hard porcelain, sanitaryware, and structural clay products). Mullite has attracted increasing interest due to its excellent high-temperature strength and creep resistance, good chemical and thermal stability, low thermal expansion coefficient, and good dielectric properties. Mullitization has been a subject of extensive and controversial investigations. This book comprehensively covers the synthesis and six types of phase transformation of mullite. Part I reviews previous research on the synthesis of mullite gels, advantages and disadvantages of different chemical routes of synthesis, and phase transformation processes. Part II discusses the nature and characterization of spinel and mullite phases and different mechanisms of mullite formation, as conjectured by various researchers. Part III deals with the critical analysis of the spinel and mullite phases and evolution of mullite formation routes. Every chapter is accompanied by detailed diagrams and a comprehensive list of references.