Adults and teenagers with poorly controlled asthma will be recruited and randomly assigned to one of two groups. Those in the mobile phone group will monitor their asthma daily using their mobile phone to record symptoms, medication and lung function. Instantaneous feedback to their phone will provide a visual indication of asthma control and prompts about therapy. The patient and their clinician will have web-based access to all readings. People in the control group will use traditional paper-based monitoring. Under the care of their asthma nurse, both groups will be treated according to the step-wise approach of the BTS/SIGN asthma guideline in order to gain control.