This research was conducted to estimate the true digestible Trp (dTrp) requirements in nursery, growing, and late finishing pigs and the effects of supplemental Trp on physiology, behavior, and meat quality. Five experiments were conducted to estimate the dTrp requirement in nursery pigs. Using broken-line regression analysis, dTrp requirements were 0.21, 0.20, and 0.18% for Phase I (5.2 to 7.3 kg), II (6.3 to 10.2 kg), and III (10.3 to 15.7) nursery pigs. In addition, four experiments were conducted to estimate the dTrp requirements in growing and finishing pigs. Using broken-line regression analysis, the dTrp requirement of pigs weighing 30.9, 51.3, and 74.6 to 104.5 kg was 0.167, 0.134, and 0.102%, respectively. An experiment also was conducted to determine the ratio of Trp:Lys and Thr:Lys in diets for nursery pigs (7.1 to 15.6 kg BW). The treatments were arranged in a 3 x 3 factorial with three ratios of true digestible Thr:Lys (0.55, 0.60, or 0.65) and three ratios of true digestible Trp:Lys (0.145, 0.170, or 0.195). Overall, optimal performance was in pigs fed the true digestible Trp:Lys ratio of 0.195 at Thr:Lys ratios 0.60 and 0.65. These results indicate that dietary levels of Trp above 0.19% may be needed to maximize growth performance in diets containing wheat and barley. Four experiments were conducted to determine the effects of supplemental Trp on meat quality and plasma and salivary cortisol and plasma lactate in growing pigs. Pigs fed the diet with supplemental Trp had lower (P < 0.01) mean plasma cortisol and lactate (P < 0.07) concentration than pigs fed the basal diet. Meat quality effects varied, but overall, results indicated that Trp had no positive effect on meat quality. Lastly, a study was conducted to evaluate the effects of Trp on growth, behavior, intestinal morphology, and brain and plasma metabolites subsequent to weaning and mixing. Cortisol was decreased (P < 0.07) after mixing in pigs fed Trp. Brain metabolites also were increased (P < 0.09) by Trp. Tryptophan supplementation has varied effects on growth performance, behavior, physiology, and meat quality in pigs.