Acute respiratory infections are the leading cause of morbidity and mortality among infants and children in developing countries. It is estimated that pneumonia causes up to 2 million deaths per year in children under 5 years of age, and neonatal pneumonia or sepsis causes an additional 1 million deaths per year [1]; an important contributing factor to these deaths is malnutrition [2]. Acute respiratory infections include both acute upper-respiratory infections and acute lower-respiratory infections. Acute lower-respiratory infections consist primarily of pneumonia, but also include croup, tracheobronchitis, and bronchiolitis. The specific aims of this chapter are to present current knowledge regarding the epidemiology, pathophysiology, diagnosis, and treatment of acute lower-respiratory infections and the potential role of nutrition in treatment and prevention.