Height control of greenhouse crops is an important practice to optimize efficient handling and rapid establishment in the field. Many techniques are available, but chemical height control has been the standard practice in commercial operations. Because of potential health risks to consumers and concerns of environmental pollution, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) have imposed restrictions on the use of growth regulating chemicals in agriculture. Use of daminozide (Alar), once the primary chemical used for controlling vegetable transplant height has been banned in the United States. As a result, no chemical growth regulators are currently labeled for height control of vegetable transplants in the United States. Growers in other countries are facing similar restrictions on using chemical growth regulators on food crops. Several research teams around the world are investigating alternative height control measures, such as gene manipulation, greenhouse temperature management, mechanical conditioning, and light quality manipulation.Follow the links to the left for information on light and plant growth, liquid spectral filters, plant responses to liquid spectral filters, physiological basis of light quality responses, commercialization of spectral filter technology, development of photoselective covers, and current concerns with photoselective films.