Gatterman developed two methods for introducing the aldehyde group into aromatic compounds. The first of these known as the Gatterman Koch reaction uses a mixture of carbon monoxide and hydrogen chloride in the presence of a mixture of anhydrous aluminum chloride and cuprous chloride. It is not adaptable to the preparation of aldehydes from phenols or phenolic ethers, however. The second method employs a mixture of hydrogen cyanide and hydrogen chloride with or without a catalyst and permits the introduction of an aldehyde group into phenols, naphthols, and their ethers under special conditions into aromatic hydrocarbons and related compounds. This chapter is concerned with the second method. The mechanism of the reaction appears to be complex and has not been fully elucidated. Details on the scope, limitations and alternative methods, as well as experimental tests, are detailed.