Development of standards and guidelines for performance specifications and testing for fibre optic sensors has been discussed since the mid-nineties of the last century in the scientific community as well as in the industry. Very global standards for the use of fibre optic components in data communication and telecommunication have been available for more than 20 years. Guidelines or substantial standards for fibre optic sensors are rather an exception. The first standard draft on generic specification of fibre optic sensors has been published in 1995 (IEC 61757-1:1995); the first draft for a specific type of fibre sensor - the fibre optic gyroscope - was published in 1996 (IEEE Standard Specification Format Guide and Test Procedures for Single-Axis Interferometric Fiber Optic Gyros; Working Draft P952/D24). Some terms used in fibre optic communication are quite close to the terminology typically used in fibre optic sensor technology. However, there are a huge number of specific issues associated with specifically fibre optic sensing systems. These items are not considered in existing guidelines or standards. For instance, standards for fibre optic sensors have to cover characteristic details related to the respective physical sensor mechanism, to the sensor response for different measurands, to the application, and finally to specific environmental conditions. Naturally, it is not possible to cover either all different aspects of fibre optic sensors in one standard or a set of harmonized standards. This very complex matter requires specific guidelines for specific sensor types (e.g. distributed sensors, point sensors such as fibre Bragg grating (FBG) sensors, sensors for mechanical measurands such as strain, deformation, biological and chemical sensors or sensors for physical quantities such as pressure, humidity, and ionizing radiation). In order to define clear guidelines and/or regulations for appropriate characterization of performance specifications and better understanding of frequently used fibre optic sensors, particular activity has been established within the European COST Action 299 “FIDES” (Optical Fibres for New Challenges Facing the Information Society) in the framework of its Working Group 4: “New Challenges in Fibre Optic Sensors”.