Lignin is currently an under-used material of the pulping and cellulosic ethanol production plants. The first and foremost objective in dealing with lignin is to augment the use of this aromatic compound and financial profitability of lignin-based processes. Of particular interests are lignin oxidation and depolymerization methods that harvest either the aromatic subunits of polymers or monomeric/oligomeric aromatic products used as platform chemicals. In this context, many studies have focused on the catalytic processes in which a nitroxyl catalyst, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), was employed to oxidize hydroxyl groups in lignin structure and selectively depolymerize lignin via cleavage of the ether and C-C bonds. In discussing the advanced studies especially over the last decade on the oxidation of lignin models with TEMPO catalyst systems, this review provides a description of promising methods that can be employed for authentic lignin conversion. In the present review, a particular emphasis is dedicated to the proposed mechanisms for the oxidation of lignin and lignin models. (C) 2019 Elsevier Ltd. All rights reserved.