Respiratory tract infections (RTI) are the leading cause of death in low-income countries and the second leading cause of death worldwide in children less than 5 years old. Most RTI are viral. Human metapneumovirus (hMPV) was discovered in 2001 in routine viral cultures of respiratory specimens from children with RTI and has been implicated as a common cause of RTI in children and adults and a cause of severe disease in immunocompromised hosts. This article describes the microbiology, epidemiology, clinical presentation, pathogenesis, diagnosis, treatment, prognosis, long-term outcome, immunity and reinfection of hMPV.