Obesity is a significant public health issue due to its prevalence and contribution to morbidity and mortality. Obesity prevention efforts in early life are important both because obesity has profound impacts on health status in childhood, and because obesity tracks from childhood to adulthood. The first thousand days in particular – from conception through age two years – may represent a critical period for obesity prevention because taste preferences and metabolic processes are shaped during this period. As such, infant feeding practices – including breastfeeding and complementary feeding practices – may be important predictors of obesity risk. However, there are significant gaps in our knowledge of the relationship between infant feeding practices and childhood obesity. The studies in this dissertation used longitudinal data from the WIC Infant and Toddler Feeding Practices Study-2 (WIC ITFPS-2) to understand the relationship between infant feeding practices - including breastfeeding duration and intensity, timing of introduction of complementary foods, and early introduction of sugar-sweetened foods and beverages - on weight trajectories and odds of overweight and obesity among low-income children up to age 3 years. Study 1 examined the relationship between breastfeeding duration and weight trajectories from age 6 months to 3 years and odds of overweight/obesity at ages 2-3 years. Mixed-effects regression models indicated that any breastfeeding (exclusive or partial) for at least 6 months and exclusive breastfeeding for at least 3 months were associated with healthier weight trajectories among both boys and girls. Breastfeeding was associated with significantly reduced odds of overweight/obesity for girls but not boys. Compared to girls who were never breastfed, those who were breastfed at all for less than 6 months had 45% lower odds of overweight/obesity while those breastfed at all for 6 months or longer had 65% lower odds of overweight/obesity. Compared to girls who were never exclusively breastfed, girls exclusively breastfed for less than 3 months had 46% lower odds of overweight/obesity and those exclusively breastfed for 3 months or more had 78% lower odds of overweight/obesity. There was evidence of a dose-response relationship between breastfeeding duration and intensity among girls. Compared to girls who were never breastfed, those breastfed for less than 3 months (short duration) had 39% lower odds of overweight/obesity, those partially breastfed for at least 3 months (longer duration, low intensity) had 55% lower odds of overweight/obesity, and those exclusively breastfed for at least 3 months (longer duration, higher intensity) had 79% lower odds of overweight/obesity. Study 2 examined the relationship between timing of introduction of complementary foods and weight trajectories from age 6 months to 3 years and odds of overweight/obesity at ages 2-3 years. Mixed-effects regression models indicated differences in weight trajectories by timing of introduction of complementary foods which varied by sex. Compared to complementary food introduction at 4-6 months, late introduction to complementary foods (at or after 7 months) was associated with less healthy weight trajectories among boys, while early introduction (<4 months) was associated with less healthy weight status among girls. However, there was no difference in odds of overweight/obesity at ages 2-3 years by timing of introduction of complementary foods among boys or girls. There was similarly no evidence that the effect of timing of introduction of complementary foods on odds of overweight/obesity differed by breastfeeding duration. Study 3 examined the relationship between early introduction to sugar-sweetened foods and beverages and weight trajectories from age 6 months to 3 years and odds of overweight/obesity at ages 2-3 years. Mixed-effects regression models indicated no difference in weight trajectories or odds of early childhood overweight/obesity between children who received soda, other sugar-sweetened beverages, or sweets such as cake or cookies before 1 year of age compared to those who didn’t receive these sugar-sweetened foods and beverages until 1 year of age or later. The three studies contribute to our understanding of the relationship between infant feeding practices and weight trajectories in early childhood. These findings suggest that longer durations of breastfeeding may be beneficial in establishing healthy weight trajectories and preventing overweight/obesity in early childhood and have important implications for policy and practice. Key policy, systems, and environmental change strategies that may improve breastfeeding initiation and duration among low-income mothers in the U.S. include a) adoption by Local WIC Agencies of evidence-based, culturally tailored participant education focused on breastfeeding promotion; b) an increase in the proportion of hospitals with Baby Friendly designation; c) paid family leave for 12 weeks following the birth of a child; and d) expansion of workplace lactation accommodation policies to cover all mothers in the workplace. Ultimately, improving breastfeeding outcomes among mothers and infants participating in the WIC program may significantly reduce the racial/ethnic and socioeconomic disparities in childhood obesity in the U.S.