ABSTRACT: BACKGROUND:Poor early-life nutrition is a major barrier to good health and cognitive development, and is a global health priority. Alive & Thrive (A&T) was a multi-pronged initiative to improve infant and young child feeding behaviors. It aimed to achieve at-scale child health and nutrition improvements via a comprehensive approach that included nutrition counseling by health workers, policy change, social mobilization and mass media activities. This study evaluated the sustainability of activities introduced during A&T implementation (2009-2014) in Bangladesh and Vietnam. METHODS:This was a mixed methods study that used a quasi-experimental design. Quantitative data (surveys with 668 health workers, and 269 service observations) were collected in 2017; and analysis compared outcomes (primarily dose and fidelity of activities, and capacity) in former A&T intervention areas versus areas that did not receive the full A&T intervention. Additionally, we conducted interviews and focus groups with 218 stakeholders to explore their impressions about the determinants of sustainability, based on a multi-level conceptual framework. RESULTS:After program conclusion, stakeholders perceive declines in mass media campaigns, policy and advocacy activities, and social mobilization activities - but counseling activities were institutionalized and continued in both countries. Quantitative data show a persisting modest intervention effect: health workers in intervention areas had significantly higher child feeding knowledge, and in Bangladesh greater self-efficacy and job satisfaction, compared to their counterparts who did not receive the full package of A&T activities. While elements of the program were integrated into routine services, stakeholders noted dilution of the program focus due to competing priorities. Qualitative data suggest that some elements, such as training, monitoring, and evaluation, which were seen as essential to A&T's success, have declined in frequency, quality, coverage, or were eliminated altogether. CONCLUSIONS:The inclusion of multiple activities in A&T and efforts to integrate the program into existing institutions were seen as crucial to its success but also made it difficult to sustain, particularly given unstable financial support and human resource constraints. Future complex programs should carefully plan for institutionalization in advance of the program by cultivating champions across the health system, and designing unique and complementary roles for all stakeholders including donors.