Cystic fibrosis (CF) is an illness that makes the lungs clog up with sticky mucus. There is no cure and so treatments are used to help make the illness easier to live with. The treatment can take lots of time and can feel not very nice sometimes. When children with CF become teenagers they need to learn to take charge of their treatment. This can be difficult. Teenagers with CF want to fit in with friends and can become more upset about their illness. Their parents have to learn to let their child take charge of their illness which can be hard for parents. These issues can put strain on parent-child relationships and this can make it harder for teenagers to stick to their treatment plans. A parenting program (called Teen Triple P) has been shown to help teenagers with other illnesses (such as diabetes) to be able to stick to their treatment plans. Parents are given a booklet to work through at home which helps them to build on the skills they already have. It aims to help families to support positive parent-child relationships, to manage difficult teenage behaviours, and to teach new skills and behaviours. So far no one has done any research to see if this program helps families of teenagers with CF. This research would like to see if the Triple P program can help teenagers with CF stick to their treatment plan. Helping teenagers stick to their treatment plan will help them to live happier and healthier lives.