The present study investigated whether satisfaction and frustration of the psychological needs for autonomy, relatedness, and competence, as identified within Basic Psychological Need Theory (BPNT; Deci and Ryan, Psychol Inquiry 11:227-268, 2000; Ryan and Deci, Psychol Inquiry 11:319-338, 2000), contributes to participants' well-being and ill-being, regardless of their cultural background and interpersonal differences in need strength, as indexed by either need valuation (i.e., the stated importance of the need to the person) or need desire (i.e., the desire to get a need met). In Study 1, involving late adolescents from Belgium and China (total N = 685; Mean age = 17 years), autonomy and competence satisfaction had unique associations with well-being and individual differences in need valuation did not moderate these associations. Study 2 involved participants from four culturally diverse nations (Belgium, China, USA, and Peru; total N = 1,051; Mean age = 20 years). Results provided evidence for the measurement equivalence of an adapted scale tapping into both need satisfaction and need frustration. Satisfaction of each of the three needs was found to contribute uniquely to the prediction of well-being, whereas frustration of each of the three needs contributed uniquely to the prediction of ill-being. Consistent with Study 1, the effects of need satisfaction and need frustration were found to be equivalent across the four countries and were not moderated by individual differences in the desire for need satisfaction. These findings underscore BPNT's universality claim, which states that the satisfaction of basic needs for autonomy, relatedness, and competence represent essential nutrients for optimal functioning across cultures and across individual differences in need strength.