IEC 61757:2018 is a generic specification covering optical fibres, components and sub-assemblies as they pertain specifically to fibre optic sensing applications. It has been designed to be used as a common working and discussion tool by the vendors of components and subassemblies intended to be integrated in fibre optic sensors, as well as by designers, manufacturers and users of fibre optic sensors independent of any application or installation. The objective of this document is to define, classify and provide the framework for specifying fibre optic sensors, and their specific components and subassemblies. The requirements of this document apply to all related fibre optic sensor standards which belong to IEC 61757 (all parts). Standards of IEC 61757 (all parts) contain requirements specific to sensors for particular quantities subject to measurement, and for a particular style or variant of such a fibre optic sensor.