This chapter discusses the effect of environmental factors on the expression of photosynthetic genes. The molecular biology of photosynthesis and the mechanisms by which the expression of genes encoding photosynthetic proteins is regulated have been investigated. The maximum photosynthetic capacity of a given plant is determined by its genetic nature, its efficiency in fixing CO2, its foliar area, its interaction with the environment, and many other genetically determined characteristics; however, the percentage of this maximum capacity that it is reached on a daily basis is influenced by many environmental factors such as light intensity, water and nutrient availability, and temperature. The effects caused by environmental factors that influence photosynthesis—by far the best analyzed at the molecular level—are those triggered by light. The effect of other important factors such as nutrient deficiencies, water availability, and temperature are also investigated in the chapter.