Catalysis is of crucial importance for the chemical industry and is used to make an enormous range of products like biologically important heterocyclic compounds and fine chemicals. It is one of the fundamental pillars of Green Chemistry and considered as the most preferred and relevant technology to achieve the reduction of waste from chemical processes. Recently, poly(4-vinylpyridinium)hydrogen sulfate [P(4-VPH)HSO4] and poly(4-vinylpyridinium) perchlorate [P(4-VPH)ClO4] as solid acid catalysts have been utilized successfully in various organic transformations. The superior advantage of these present catalysts is that they can be effectively recovered and further reused up to several times without significant loss of its efficiency. In this present book, we wish to give an overview of the utilization of these solid and heterogeneous catalysts in the synthesis of various organic compounds having industrial and pharmaceutical applications. Successful green reactions are considered, and experimental and important practical details, with illustrations of potential applications, are provided throughout the book.