HHO is one of the several solutions for internal combustion engines efficiency improvement. There are many controversies and theories recommending and condemning the usage of HHO. However the current book gives an insight into what actually is HHO, the theory behind the usage of HHO, also citing ample examples of HHO usage in engines as available in the literature emphasizing the advantages of HHO usage in the engines. Also the book discusses the experiments conducted by the author and the team for the verification of benefits. Same time author addresses the claimants, who state that HHO is a spam, with a number of checks to cross verify the results before coming to conclusion. Various issues like HHO generator types, installation, cost dynamics, Etc are provide in the suitable context of the book. At the end of the book, a number of links are provided for interested readers to have a glance of the products(HHO generators) available in the market, to compare the features and to select for their applications. The book is a wonderful collection for common public, Engineers, Researchers, Students, Manufacturers, and for any one interested in technology.