Recommendation ITU T G.697 defines optical monitoring (OM) that can help in dense wavelength division multiplexing (DWDM) systems to perform the following activities: - configuration management for system and channel activation, addition of new channels, etc.; - fault management to detect and to isolate faults; - degradation management in order to keep the system running and to detect degradations before a fault occurs. DWDM technology is improving at a rapid pace, continuously stretching the channel count, channel speeds and reach limits. Long haul multi-span DWDM systems are capable of taking optical signals thousands of kilometres without electrical terminations or regeneration. This continuing trend is driving the increasing importance of optical monitoring, which is the subject of this Recommendation. This edition of this Recommendation provides additional information on optical monitoring for 40 Gbit/s and 100 Gbit/s signals, adds beat noise method to the clause for optical signal to noise ratio (OSNR) measurements, and introduces a new clause on security considerations.