This review summarises the effects of giving deworming drugs to children to treat soil transmitted intestinal worms (nematode geohelminths) on weight, haemoglobin, and cognition; and the evidence of impact on physical well being, school attendance, school performance, and mortality. It was found that the main soil-transmitted worms are roundworms, hookworms, and whipworms. Infections are common in tropical and subtropical areas, particularly in children from low-income areas where there is inadequate sanitation, overcrowding, low levels of education, and lack of access to health care. These infections may cause malnutrition, poor growth, and anaemia in children, and some experts believe they cause poor performance at school. Improvements in water and sanitation have been shown to be vital, but drugs for deworming might play a role too. In one approach, individuals found to be infected on screening are treated. Evidence from these trials suggests this probably improves weight and may improve haemoglobin values, but the evidence base is small. In another approach, currently recommended by the WHO, and much more extensively investigated, all school children are treated. In trials that follow up children after a single dose of deworming, and after multiple doses with follow up for over a year, we do not know if these programmes have an effect on weight, height, school attendance, or school performance; they may have little or no effect on haemoglobin or cognition.