The creation of environments that are more supportive of optimal infant and young child feeding (IYCF) requires countries to enact policies, such as those related to the Maternity Protection Convention, the International Code of Marketing of Breast-milk Substitutes (the Code), and the Baby-Friendly Hospital Initiative. However, challenges are experienced in the translation of international policy standards into national legal measures, and there is an important gap in understanding how countries achieve progress. Policy advocacy is a nearly universal feature, but there are methodological challenges and few studies evaluating strategies and effects. The purpose of this supplement to Maternal & Child Nutrition is to address those gaps. This supplement contains three papers that present findings from a real-time evaluation of the advocacy efforts of Alive & Thrive (A&T), United Nations International Children's Emergency Fund (UNICEF), and partners, that sought to support governments in fostering enabling environment for optimal IYCF in Southeast Asia (SEA) and Africa. A combination of two emergent, theory-based evaluation approaches was used: developmental evaluation and contribution analysis. The overall objective of the evaluation was to document the extent to which policy objectives were or were not achieved in each country and to identify the key drivers of policy change. One contribution of the supplement is a distinction between and illustration of triggers and drivers of policy change. Three main drivers of policy change were identified: (a) the use of an explicit advocacy approach; (b) the creation of a strategic group of actors; and (c) the realization of 15 critical tasks (more specifically for the Code). Each of the critical tasks has been identified as having triggered progress on the Code in those countries. This supplement provides evidence that the advocacy efforts of A&T, UNICEF, and partners contributed to enhanced IYCF policies in SEA and reveals how it helped to achieve progress. The insights contained in this supplement can serve as a guide for policy advocates for enhanced IYCF policies. A short communication puts findings into perspective within global context.