Technical Abstract: Ancestors of commercial cotton varieties are of tropical origin, and are naturally adapted to growth in warm environments. Today, the derived cultivars retain the high optimal temperatures for growth that are characteristic of their progenitors. It is valuable to review the reported optimal temperature(s) for cotton before examining plant responses to temperatures extremes. Despite many studies evaluating cotton temperature responses during the past century, a clear picture has yet to emerge whether there is a single temperature optimum for cotton metabolism. Some reports suggest distinct temperature responses within different anatomical structures, or at diverse developmental times. Discrepancies between reported temperature responses can be related to the measure of temperature itself. Some studies describing optimal temperatures for cotton refer to air temperature, while others refer to plant temperature. The discussion below will show that air and plant temperature measurements cannot be used interchangeably. This chapter will provide brief overviews of temperature measurement techniques, reported temperature optima, and the effects of low and high temperature stresses on cotton metabolism.