Recommendation ITU-T G.874 addresses management aspects of optical transport network (OTN) elements containing transport functions of one or more of the layer networks of the OTN. The management of the optical layer networks is separable from that of its client layer networks so that the same means of management can be used regardless of the client. The management functions for fault management, configuration management and performance monitoring are specified. Recommendation ITU-T G.874 (2008) updated the management information (MI) to align with Recommendation ITU-T G.798, reorganized the sections to align with the structure of Recommendation ITU-T G.7710/Y.1701, and replaced the generic text with pointers to Recommendation ITU-T G.7710/Y.1701. Recommendation ITU-T G.874 (2010) added the management of new transport functions that were introduced in ITU-T G.798 (2010), including OPSMnk_TT, OPSM/OTUk a_A, and ODUk for k=0, 2e, 4, and flex. Recommendation ITU-T G.874 (2013) added the management of hitless adjustment of ODUflex (GFP) (HAO), APS, Application codes and performance management (PM) data collection. Recommendation ITU-T G.874 (2017) adds description to cover OTUCn GCC0, adds the application code related MI signals, updates the MI signals for ODU2eP/FC-1200_A, OSM256.4/CBRx_A, OSx/CBRx-b_A_Sk and OSx/CBRx-c_A_Sk, removes the nDelay, nES, and fES primitives, moves the description of O.MN, O.MSN, and O.NE to the convention clause, and updates the default values of DEGThr and DEGM, updates Appendix III to align with Table 15-9 of ITU-T G.709/Y.1331 (/2016), and removes the adaptation function activation and MI_Active to align with ITU-T G.798. Recommendation ITU-T G.874 (2020) aligns with the updates in ITU-T G.709 and ITU-T G.798, and harmonizes with ITU-T G.7710 clauses 8 and 10 on the generic requirements.