A drive interface structure 706 interfacing a surgical robot arm and a surgical instrument. The drive interface structure 706 comprises a sheet of flexible material 1002 providing a barrier and a rigid rim 1004 attached to an outer edge of the sheet of flexible material 1002. The rigid rim 1004 is configured to engage an interface structure. The drive interface structure may comprise a gasket forming a tight seal. An interface structure for detachably interfacing a surgical robot arm and instrument is also provided. The interface structure comprises a base portion comprising a rim surrounding an aperture through which drive assembly interface elements transfer drive to interface elements of the instrument. The interface structure comprises one or more protrusions supported by the base configured to receive a drive interface structure comprising a sheet of flexible material. An interface assembly comprising the above drive interface structure and interface structure and a surgical robot comprising the interfacing portions are also provided.
