Purpose: This study aimed to examine the longitudinal associations of person-related factors with physical activity (PA) behavior in young adults. Methods: We analyzed longitudinal self-reported time spent in moderate-intensity PA (MPA; 4-7 METs) and vigorous-intensity PA (VPA; >7 METs) from 499 young adults (49% male) who participated in the Amsterdam Growth and Health Longitudinal Study at the age of 21, 27, 32, and 36 yr. Sociodemographic factors (i.e., marital and employment status), physical factors (i.e., skinfolds, aerobic fitness, neuromotor fitness, back problems, and general health status), psychological factors (i.e., problem and emotion focused coping, mild health complaints, and personality), and behavioral factors (i.e., alcohol consumption, smoking, and energy intake) were assessed at each time point. We performed sex-specific univariable and multivariable generalized estimating equations. Results: Men and women with higher aerobic fitness were more moderately and vigorously active. Not having paid work was associated with more MPA in both men and women. Men with part-time paid work, lower scores on dominance, higher scores on hostility, and above moderate alcohol consumption (i.e., ≥140 g of alcohol per week) were more moderately active. Divorced women and those with better physical flexibility spent more time in MPA. Men having full-time paid work, with a good general health status and nonsmokers, were more vigorously active. Women being married/living together, who had better physical flexibility, lower scores on inadequacy, higher scores on dominance, and low caloric intake (around 2000 kcal·d) were more vigorously active. Conclusion: Several sociodemographic, physical, psychological, and behavioral factors were associated with PA in Dutch young adults. Determinants were different for MPA and VPA and for men and women. © 2014 by the American College of Sports Medicine.