IEC 61400-4:2025 is applicable to enclosed speed increasing gearboxes for horizontal axis wind turbine drivetrains with a power rating in excess of 500 kW. This document applies to newly designed gearboxes for wind turbines installed onshore or offshore. The technical requirements given in this document are not intended for repaired or refurbished gearboxes, or for the extension of the service life beyond the design life. This document provides requirements and guidance on the analysis of the wind turbine loads in relation to the design of the gear and gearbox elements. The gearing elements covered by this document include such gears as spur, helical or double helical and their combinations in parallel and epicyclic arrangements in the main power path. This document does not apply to power take off (PTO) gears. This document includes requirements, design recommendations, and rating of gearboxes with rolling bearings, plain bearings, or combinations of both bearing types. This document is supported by two Technical Reports: IEC TR 61400‑4‑2 provides additional information on lubrication of wind turbine drivetrains and IEC TR 61400‑4‑3 contains explanatory notes and supportive information to the requirements specified in this document. It is published as a double logo standard. This second edition cancels and replaces the first edition published in 2012. This edition includes the following significant technical changes with respect to the previous edition: - extension of the scope to wind turbines above 2 MW reference power; - considerations for converging differing approaches to reliability in gear, bearing and wind turbine standards; - new clause on wind turbine loads specific to drivetrains; - revised clause on verification and validation; - new clause on design requirements for plain bearings; - revised and expanded design considerations for rolling bearings; - revised clause on considerations and requirements in the design and analysis of gearbox structural elements; - updated considerations and requirements on lubricants and lubrication systems; - removal of requirements for documenting the compliance of a design with the requirements of the document in favour of reference to IECRE OD‑501‑2.