BACKGROUND: Globally, some 4.7 million infants aged under 6 months are moderately wasted and 3.8 million are severely wasted. Traditionally, they have been over-looked by clinicians, nutritionists, and policy makers. OBJECTIVE: To present evidence and arguments for why treating acute malnutrition in infants under 6 months of age is important and outline some of the key debates and research questions needed to advance their care. METHODS: Narrative review. RESULTS AND CONCLUSIONS: Treating malnourished infants under 6 months of age is important to avoid malnutrition-associated mortality in the short-term and adverse health and development outcomes in the long-term. Physiological and pathological differences demand a different approach from that in older children; key among these is a focus on exclusive breastfeeding wherever possible. New World Health Organization guidelines for the management of severe acute malnutrition (SAM) include this age group for the first time and are also applicable to management of moderate acute malnutrition (MAM). Community-based breastfeeding support is the core, but not the sole, treatment. The mother-infant dyad is at the heart of approaches, but wider family and community relationships are also important. An urgent priority is to develop better case definitions; criteria based on mid-upper-arm circumference (MUAC) are promising but need further research. To effectively move forward, clinical trials of assessment and treatment are needed to bolster the currently sparse evidence base. In the meantime, nutrition surveys and screening at health facilities should routinely include infants under 6 months of age in order to better define the burden and outcomes of acute malnutrition in this age group.