This book provides an in-depth introduction to the sol to gel transition in inorganic and hybrid organic-inorganic systems, one of the most important chemical-physical transitions and the basis of the sol-gel process. Familiarity with the fundamental chemistry and physics of this transition is essential for students in chemistry and materials science through academic and industry researchers working on sol-gel-related applications. The book features a didactic approach, using simple and clear language to explain the sol to gel transition and the accompanying processes. The text is also suitable for use in short courses and workshops for graduate students as well as professionals. This fully revised and updated new edition contains a wealth of new content. In particular, it includes a detailed discussion of the chemistry of transition metal alkoxides and organosilanes, and an extended discussion of the sol to gel transition models.