High temperature stress is an important yield limiting factor in the arid and semi-arid regions of the world. Our research on grain crops (sorghum, rice, soybean and peanut) have shown that various reproductive processes (pollen viability and germination) are sensitive to high temperature stress leading to lower seed-set and grain yield. Objectives of our research were to (i) understand the impact of high temperature stress on reproductive processes, and (ii) identify physiological and biochemical bases of loss of pollen viability under high temperature stress. Controlled environment experiments were conducted to study the impact of high nighttime temperatures on reproductive processes, ultrastructure and lipid profile of pollen grains. High nighttime temperatures (28 vs. 22 degree C for sorghum; and 29 vs. 20 degree C for soybean) significantly decreased pollen viability and seed-set in both sorghum and soybean. Under high nighttime temperatures, pollen grains of sorghum had higher production of reactive oxygen species (ROS) and greater membrane damage. Analyses of phospholipids in pollen grains of sorghum and soybean showed that under high nighttime temperature stress decreased phosphatidyl glycine (PG) and phosphatidic acid (PA) level and increased phosphatidyl choline (PC), and lysoPG and lysoPC and unsaturation level of phospholipids. Decreases in PA and saturation level of phospholipids along with increases in ROS and membrane damage, indicates oxidative stress in pollen grain. These changes in phospholipids and ROS may be responsible for loss of pollen viability under high temperatures stress. We are currently testing if these changes occur across other crop species. In addition, it is important to test if genotypes vary in their tolerance or susceptibility to high temperatures. Identification of genes and understanding of mechanisms of tolerance will help in development of high temperature tolerant genotypes of grain crops.