A seal for use in a surgical instrument to provide a gas-tight seal with an instrument passed through the seal. The seal can form a gas-tight seal with an instrument having a diameter within a wide range of diameters. The seal comprises a seal body, an instrument seal, and a laterally-compliant seal mounting. The seal body includes a bore through which the instrument is passed. The instrument seal is made of an elastic material. The instrument seal extends radially outwards from an instrument port formed in the instrument seal through which the instrument is passed. The instrument port is substantially perpendicular to the axis. The instrument seal also extends axially from the instrument port in the direction opposite to that in which the instrument is passed through the instrument port. The laterally-compliant seal mounting mounts the instrument seal to the seal body, forms a gas-fight seal between the instrument seal and the seal body, and allows the instrument seal to move freely laterally in response to lateral movement of the instrument.