All electric lighting systems degrade in performance during their operating life. Light sources produce less and less light the longer they operate, while their spectral output may shift in color. Light output and distribution may be affected by dirt and dust accumulation on luminaire surfaces. Components fail over time. To ensure continuing operations, owners take actions to preserve the functionality of their lighting systems, referred as maintenance. Well maintained lighting systems function in a predictable manner while offering the potential for capital and operating cost savings. Poorly maintained lighting systems can lead to problems with user satisfaction, productivity, aesthetics, sales, safety and security, and the ability to capture intended energy savings on an ongoing basis. This document examines common lighting maintenance procedures and the central components of an effective planned maintenance program.