Respiratory viruses such as respiratory syncytial virus (RSV), parainfluenza viruses (PIV) and the influenza viruses cause severe lower respiratory tract disease in infants and children throughout the world. We discuss the recent discovery of the epidemiologic importance of the human metapneumoviruses, first reported in 2001. Experimental live-attenuated vaccines for each of these viruses are being developed for intranasal administration in the first weeks or months of life. The immunology of these infections in humans is poorly defined but many studies are ongoing. A significant obstacle to successful immunisation of infants against respiratory-virus-associated disease early in life may be the relatively poor immune response of very young infants to primary virus infection. This paper reviews the immune correlates of protection against disease caused by these viruses, immune responses of infants to naturally acquired infection or experimental infection with candidate vaccine viruses and the genetics of susceptibility to severe disease.