The purpose of this Recommendation is to indicate a minimum@ but not exhaustive@ set of optical parameters that can be used to perform optical monitoring (OM) functions in dense wavelength division multiplexing (DWDM) systems and optical network elements [ONEs@ e.g.@ reconfigurable optical add-drop multiplexers (ROADMs)]@ particularly relevant to those network elements without optical-electrical-optical conversions. In order to achieve this objective@ this Recommendation: 1) indicates methods for measuring the optical signal degradation; 2) classifies those methods by type; 3) defines suitable optical parameters to detect optical signal degradation; and 4) describes the applications or conditions where these optical parameters can be relevant. This Recommendation refers to DWDM systems and ONEs with optical channels with bit rates up to approximately 10 Gbit/s using non-return to zero (NRZ) or return to zero (RZ) line coding and bit rates at approximately 40 Gbit/s and 100 Gbit/s using advanced modulation formats@ such as (dual polarization) quadrature phase shift keying. Bit rates above 100 Gbit/s and systems employing other modulation formats are for further study.