Greater urbanization and higher income are risk factors for obesity in urbanizing countries, likely due to the adoption of obesity-related dietary behaviors and decreased physical activity (PA). Yet, to what extent urbanicity and income jointly influence BMI indirectly through caloric intake and PA has not been examined in adults experiencing urbanization. In this dissertation, we capitalized on longitudinal data from The China Health and Nutrition Survey (CHNS; >19,000 individuals ages 18-75y seen up to 7 times between 1991 and 2009). We investigated time-varying associations for income, urbanicity, and the interaction between urbanicity and income with key markers of Westernized diet [total kcals and percent calories from animal-source foods (%animal-source kcals)], PA domains (occupational, domestic, travel, leisure, and total PA), and BMI over 18 years. We constructed a pathway-based structural equation model (SEM) to examine indirect pathways from urbanicity and income through total kcals and total PA to BMI over time. From statistical models, large behavior changes in less urban areas led to dietary and PA behaviors becoming more similar to those observed in individuals in more urban areas over time. Higher income was associated with higher total kcals and %animal-source kcals over time for individuals in lower urbanicity areas (p<0.05), but not in higher urbanicity areas. Higher income was associated with higher total PA for individuals at all urbanicity levels, particularly at later time points (p<0.05). From our pathway-based SEM, adjusted mean BMI increased over time (p<0.05) for men at all urbanicity levels, but only for women living in lower urbanicity areas. Living in a higher versus lower urbanicity area was indirectly associated with higher BMI through lower PA (p<0.05) and was associated with lower BMI through lower total kcals (p<0.05). In men, but not women, estimated indirect associations from urbanicity to BMI through PA were larger in magnitude than those through total kcals (p<0.05). Our findings suggest that addressing Westernized dietary behaviors and low PA for individuals in less urban areas is critical early in the urbanization process. In men, the predominant pathway from living in a higher urban area to higher BMI was through lower PA, suggesting that PA is a critical intervention target for men in highly urbanized areas. The difference in mean BMI by urbanicity narrowed in women over time due to higher caloric intake in less versus more urban areas at all time points, highlighting the importance of dietary intervention for women in less urban areas.