The IEC 61400-8 outlines the minimum requirements for the design of wind turbine nacelle based structures and is not intended for use as a complete design specification or instruction manual. This standard focuses on the engineering integrity of the structural components constituted within and in the vicinity of the nacelle, including the hub, mainframe, main shaft, associated structures of direct-drives, gearbox structures, yaw structural connection, nacelle covering and other structural connections to subsystems of control and protection mechanisms, electrical units and mechanical systems. The standard focuses primarily on Ferrous material based nacelle structures, but can apply to other materials also as appropriate. The design of bolted and welded joints in the nacelle structures is included, as well as cast and forged components. Material testing requirements to use in the design process for nacelle structures are specified. Structures or components outside of the nacelle, such as the tower, foundations or blades are not in the scope of this standard. Further, the standard does not address non structural components or systems such as hydraulics or electrical units. While the structural connections of the gearbox and the main shaft, are in scope, the design of the gears and bearings are not included. This standard shall be used together with the appropriate standards mentioned in Section 2. In particular, this standard is consistent with the requirements of IEC 61400-1 Ed.4. The safety level of the wind turbine designed according to this standard shall be at or exceed the level inherent in IEC 61400-1 Ed.4. Probabilistic methods to calibrate partial safety factors and for use in the design process are provided.