Greenhouses harvest light and the way growers use this light set us apart from any other production system and allows us enormous opportunities to steer our crops to an optimised vegetative/generative balance for enhanced yield, quality and produce uniformity. Plants look at light in a very different way to humans and we need to understand these differences and the opportunities it affords us to better meet the needs of plants and how to make them comfortable and productive. We say that 1% light equals 1% production therefore we must strive to optimise light at all times using a wide range of tools that are available to us and understand its effects on plant physiology. (how plants grow in a greenhouse and what affects its growth!) This presentation will explore light characteristics, the impacts on plant performance and some tools to influence the light intensity and sum.