This book covers various subtopics of catalyst research. It provides essential background material on key concepts in catalyst technologies, along with recent developments in synthesis methods.  The book delves into diverse areas such as photocatalysis, electrocatalysis, biocatalysis, and heterogeneous catalysis, offering a detailed exploration of their respective advancements, advantages, disadvantages, and applications in different catalytic reactions. It also presents fundamental ideas and overviews of noble-metal-free bifunctional electrocatalysts for overall water splitting in alkaline medium, catalytic nitrite reduction, and the utilization of catalysts supported on various materials like CeO2, Nb2O5, ZrO2, TiO2, γ-Al2O3, SiO2, and ZSM-5 for the removal of high concentrations of nitrite in water.  Moreover, the book provides insights into topics such as polyphenol oxidase mimics as catalysts, ferrites nanoparticles/graphene oxide nanoparticles and their composites as enzyme mimics and photocatalysts, electrochemical approaches with basic concepts and materials for hydrogen technologies, as well as recent advances and applications of modified-semiconductor photocatalysts in pollutant degradation.  For scientists and researchers, the book offers a wealth of recent experimental and computational results, along with in-depth discussions. Some of the specific areas covered include sphalerite ZnS nanoparticles (NPs) synthesized via solvothermal method, incorporating various ruthenium (Ru) concentrations. Additionally, the book explores the modification of TiO2 as SO4/TiO2 acid and CaO/TiO2 base catalysts, and their applications in converting waste frying oil into biodiesel.