The main objective of this book is to provide the basic concepts about polyimides, which fall under the category of high performance and high temperature polymers. This would be useful to researchers who work in the field of polyimides, especially in microelectronic applications. The first chapter gives the basic introduction about polyimides and its composites. The second chapter details the experimental procedures and testing methods involved in the synthesis of polyimides and its composites. The third and fourth chapter discusses the synthesis and characterization of two systems of homo, co and terpolyimides. The fifth chapter compares the mechanical, thermal and dielectric properties of the two systems to choose a polymer with low dielectric constant and good mechanical and thermal properties. The sixth chapter discusses the synthesis and characterization of mesoporous silica. The seventh chapter emphasizes on the incorporation of mesoporous silica into the polyimide matrix to further reduce the dielectric constant without compromising the mechanical and thermal properties, so that the mesoporous silica/terpolyimide composites becomes suitable for microelectronic applications.