<p>Infant and young child feeding practices are suboptimal in Tanzania, but optimal nutrition during the first two years of life is critical for development. At this age, behaviors of caregivers determine children’s diets, but few studies quantify how fathers or couples jointly influence child feeding. We investigate how men and women shape the family food environment to determine children’s diets in rural Mara, Tanzania. This mixed methods study used quantitative baseline data from the Engaging Fathers for Effective Child Nutrition and Development in Tanzania (EFFECTS) trial (ClinicalTrials.gov Identifier: NCT03759821), including 960 rural households with both parents and a young child. Logistic and linear mixed effects regressions examined associations between parental determinants and exclusive breastfeeding (n=189) and child dietary diversity over one day and one week (n=764 children 6-23 months; n=597 children 9-23 months). Parental determinants measured knowledge of breastfeeding and complementary feeding, self-efficacy on provision of diverse or nutritious diets, workload away from home, social support, household savings, and couples’ communication and decision-making, indicating women’s empowerment. Qualitative data drew from formative research (July-August 2018) conducted with parents of children aged 0-36 months, including eight focus group discussions (FGDs) with mothers and fathers (four FGDs each, 31 and 30 participants, respectively). FGDs were transcribed, translated, quality checked, and coded to identify key themes. The odds of exclusive breastfeeding decreased with mothers’ higher workload away from home and increased with mothers’ reports of more frequent couples’ communication over household decisions. Higher child dietary diversity was linked to mothers’ reports of higher knowledge of nutritious foods, higher social support, and higher couples’ communication frequency and quality. Generally, fathers’ determinants were not associated with children’s diets. Qualitative findings indicated that knowledge of exclusive breastfeeding was common, but challenges of perceived breastmilk insufficiency, women’s workload, and concerns about breastmilk quality led to early introduction of complementary foods. Affordability was a common challenge to appropriate complementary feeding, while couples’ decision-making dynamics and attitudes and beliefs about foods had the potential to be both enablers and barriers. Interventions must improve nutrition knowledge and social support, and they must empower women through joint decision-making and more gender-equitable workloads between parents to enable optimal child diets. </p>