Breastfeeding is sustainable, the biological norm, and potentially life-saving, particularly for premature babies. Evidence-based strategies to support breastfeeding have been successful, but inequalities in breastfeeding rates are proving difficult to reduce, affecting the most vulnerable of mothers and babies. Successfully establishing and sustaining breastfeeding can be facilitated by both removing structural and cultural barriers, and overcoming individual challenges. Common factors such as obesity and depression/anxiety could play an important part in explaining some of the variability (and inequality) in breastfeeding duration. Conversely, maternal factors reflecting good mental and physical health could increase resilience to contexts with low systemic and cultural support for breastfeeding, such as the UK. However, the evidence on the individual determinants causally influencing successful and sustained breastfeeding is of poor quality. The identification of causal determinants of early cessation will improve breastfeeding support activities.