Childhood asthma is a major public health problem in New Zealand. The evidence base for the treatment of childhood asthma is considerably less than for adult asthma. In adults, treatment with a single inhaler that contains both preventer and reliever medications, used as-needed for asthma symptoms, is markedly better at reducing asthma attacks compared to the common approach of taking a reliever-only inhaler as-needed for all asthma severity. This new treatment regimen has not been studied in children with moderate and severe asthma. If comparable efficacy with as-needed preventer+reliever inhaler treatment is shown in childhood as with adult asthma, then implementation of this approach would markedly reduce the burden of asthma in children. In this randomised controlled trial, undertaken in partnership with community general practitioners, the effect of the combined budesonide+formoterol inhaler taken as-needed will be compared with salbutamol as-needed in children aged 4-11 years with moderate and severe asthma.