Asthma is a chronic condition affecting approximately 5-15% of all Swedish children. Research has shown that asthma may be associated with increased anxiety and worry, with children who have asthma being twice as likely to experience anxiety compared to those without asthma. The coexistence of anxiety and asthma can worsen asthma symptoms and heighten anxiety, as it can be challenging to distinguish between the two conditions. Cognitive behavioral therapy (CBT) is a recommended treatment for anxiety in children, but few studies have examined the effectiveness of CBT in children with both asthma and anxiety. Our research group has previously developed internet-delivered CBT (Internet-CBT) targeting anxiety in asthma for adults, yielding promising results. Internet-CBT can significantly improve access to treatment and has been shown to be both effective and safe for other medical conditions. The primary aim of this project is to investigate whether Internet-CBT can contribute to improved quality of life, better asthma control, and reduced anxiety in children and adolescents whose asthma is complicated by their own or their parents' anxiety. We have previously developed Internet-CBT for adults with asthma and anxiety with promising outcomes. For this study, we have adapted the treatment for children, adolescents, and their parents and will evaluate its feasibility and clinical effectiveness.