To ensure the proper performance of optical transmission systems@ optical return loss is measured at the component level and at the installed fiber plant. Reflected optical power contributes to the loss of optical power within the fiber optic system and in higher speed transmission systems@ in particular@ can cause interference during signal processing. In order to ensure that a fiber optic network exhibits little back reflection@ the total optical return loss of the network may be controlled and verified. Various types of instruments are used within the fiber optics industry to measure the return loss and/or reflectance of fiber optic components@ assemblies@ and systems. The purpose of this white paper is to build awareness of the performance and application issues affecting instruments used to measure optical return loss and reflectance on both the component and the system level. Different instruments may be used to measure optical return loss and reflectance of system components such as connectors@ filters@ isolators@ diodes@ subsystems@ cable assemblies@ and an entire fiber span. Under certain circumstances@ these instruments might measure the reflection of a point defect@ or the cumulative reflected power of a whole system. In order to compare the performance criteria of instruments used to measure optical return loss and to develop an understanding of their functionality and the measurement uncertainty associated with their use@ some terminology and common application issues are collected. It is the intent of this paper to combine criteria@ parameters@ and issues that help users of these test instruments understand common phenomena associated with them. In order to build awareness about common issues with using instruments used to measure optical return loss this paper addresses: * What optical return loss is *When and where optical return loss is measured * The various kinds of instruments used to measure optical return loss * Performance criteria * Calibration issues * Applications issues * The industrial status of instruments used to measure optical return loss