The conventional removal of functional groups is based on the use of stoichiometric amounts of reagents, such as in the textbook examples of the Clemmensen and Wolff- Kishner reactions {see Science of Synthesis, Vol. 48 [Alkanes (Section 48.1.3.1.3 and Section 48.1.3.1.1, respectively)]}. However, these and related methods do not meet the requirements of modern green chemistry. On the other hand, metal-catalyzed defunctionalization using a hydride source, also called hydrogenolysis, constitutes a more atom-, step-, and waste-economic method. The term hydrogenolysis refers to the cleavage of single bonds between two carbon atoms or a carbon and a heteroatom (mostly O, N, or S) using hydrogen. In a broader sense, the reduction of carbonyl compounds to the corresponding alkanes can also be included within the definition. The first reaction of this type can be dated back to the work of Padoa and Ponti on nickel-catalyzed reduction of furfural via furfuryl alcohol.[1] Later, Connor and Adkins reported a more comprehensive study on copper-chromium oxide catalyzed hydrogenolytic cleavage of C-O and C-C bonds.[2] This overview covers the hydrogenolysis of carbon-oxygen and carbon-nitrogen bonds in benzylic positions (Scheme 1). Traditionally, this transformation was mainly used in synthetic organic chemistry to remove the benzyl protecting groups from benzylamines and benzyl ethers to furnish the free amino and hydroxy groups. Hydrogenolysis of benzylic alcohols and aromatic carbonyl compounds leads to alkylarenes. More recently, the role of this transformation in the utilization of biomass feedstock has become of importance.[3-5] A typical catalyst used for these transformations is heterogeneous palladium, but also other metals, such as cobalt, nickel, ruthenium, and rhodium, are able to cleave bonds using hydrogen. Examples using homogeneous metal catalysts are also known. Many methods have been developed since the first reported hydrogenolysis, including modifications of the catalyst, reaction conditions, additives, and the reducing agent. Among these variations, particularly the latter has much impact on the practicality of the method. While hydrogen gas is an inexpensive and potentially sustainable (depending on its preparation) reductant, it is flammable and requires special equipment. Therefore, transfer hydrogenolysis, using alternative sources of hydrides, has become important. Among the various reducing agents used, formic acid, alcohols, and cyclohexa-1,4- diene are common and will be discussed in the respective sections. © 2018 Georg Thieme. All rights reserved.