The presence of pendant hydroxyl groups on the polyimide chains in a polyimide/silica system significantly modifies morphology, enhances compatibility between the phases, reduces silica network size and generates co-continuous networks structures. Compatibilizing power of pendant hydroxyl groups increases significantly when used concurrently with coupling oligomers laden with such functional groups that help in establishing either chemical bonds and/or strong physical interactions. The effect of hydroxyl groups on the silica micro-structures, compatibility, thermal and thermo-mechanical properties is presented in this book. The discussion is based on polyimide/silica and poly(hydroxyl imide)/silica hybrids containing pure as well as organically modified silica network structures prepared through sol-gel process. The link between compatibility enhancement and the origin & concentration of hydroxyl groups has also been explored. This book is useful for academics as well as researchers working in the field of polymer Nanocomposites as it contains original scientific results obtained by the author accompanied with substantial literature review.