Dense wavelength division multiplexing (WDM) transmission systems have taken on a level of sophistication never before seen in the telecom industry. Wavelength spacings of 100 GHz (0.8 nm) and 50 GHz (0.4 nm) are challenging system manufacturers to provide precise and stable wavelength control and monitoring techniques that will ensure long-term reliability. Until now, precise-wavelength distributed-feedback lasers have been able to provide the accuracy and stability that WDM systems require. As telecommunications systems put more wavelengths down each fiber, requirements increase for channel spacing and system reliability, and a real-time wavelength control system becomes a necessity. The integral part of this wavelength control technique is the wavelength locker. Invented more than 20 years ago for WDMs in multimode fiber optic systems, wavelength-locking techniques are reaching new heights of sophistication and performance capabilities for advanced system requirements.