Severe acute malnutrition (SAM) can have high mortality, especially in very ill children treated in the hospital. Many medical and nursing schools do not adequately, if at all, teach how to manage children with SAM. There is a dearth of experienced practitioners and trainers to serve as exemplars of good practice or participate in capacity development. We consider 4 country studies of scaling up implementation of WHO guidelines for improving the inpatient management of SAM within under-resourced public sector health services in South Africa, Bolivia, Malawi, and Ghana. Drawing on implementation reports, qualitative and quantitative data from our research, prospective and retrospective data collection, self-reflection, and our shared experiences, we review our capacity-building approaches for improving quality of care, implementation effectiveness, and lessons learned. These country studies provide important evidence that improved inpatient management of SAM is scalable in routine health services and scalability is achievable within different contexts and health systems. Effectiveness in reducing inpatient SAM deaths appears to be retained at scale.The country studies show evidence of impact on mortality early in the implementation and scaling-up process. However, it took many years to build workforce capacity, establish monitoring and mentoring procedures, and institutionalize the guidelines within health systems. Key features for success included collaborations to build capacity and undertake operational research and advocacy for guideline adoption; specialist teams to mentor and build confidence and competency through supportive supervision; and political commitment and administrative policies for sustainability. For frontline staff to be confident in their ability to deliver appropriate care competently, an enabling environment and supportive policies and processes are needed at all levels of the health system.