There are 9 cardinal parameters that determine plant growth and development under electric lights in controlled environments. Among these, radiation quantity is the most difficult parameter to measure accurately and to control. Most users over-worry about spectral quality and under-worry about radiation intensity. None-the-less, spectral quality has a significant effect on plant development and it is nearly impossible to replicate the solar spectrum with electric lights. Here we review the effects of UV radiation, red/far-red radiation, and narrow spectrum radiation from electric lights. Normal plant development can be achieved under a wide range of electric lamps – provided the intensity is sufficiently high.