This chapter presents data on the economic importance, flower morphology, photoperiodic control of development, sex expression, and agronomic implications of Spinacia Oleracea, along with the effect of low temperature on flower formation. Spinach as a cultivated herb originated in Iran during the times of the Persian Empire, and was introduced into China via Nepal in 647 A.D. It is grown for both the fresh market and for processing. Spinach is of modest economic importance in both North America and Western Europe. Sex expression is under genetic control in spinach, as in other dioecious plants. It should be pointed out that environmental factors and exogenous growth regulators can modify sex expression in spinach. For optimum yield of high-quality leaf material, spinach is usually harvested just as the plants are beginning to bolt.