Sleep disturbances, such as insomnia and sleep apnea, are very common following acquired brain injury and have serious consequences. Poor sleep is associated with health problems, mood disturbances and work absenteeism. This thesis focuses specifically on insomnia following a traumatic brain injury or a stroke. For people with acquired brain injury, insomnia may exacerbate all other brain damage related problems, such as cognitive and emotional disturbance, leading to poorer quality of life. Treatment is needed, but to the best of our knowledge, there are no specific guidelines for the treatment of insomnia for people with acquired brain injury yet. In the general population, cognitive behavioral therapy for insomnia (CBT-I) is treatment of choice, and has demonstrated impressive efficacy in multiple randomized controlled trials. Online versions have been developed to reach a larger group of people with insomnia, with comparable efficacy to face-to-face CBT-I. If these findings will also hold for people with acquired brain injury, this could have major benefits. The central question of this thesis is whether cognitive behavioral can be effective for people with traumatic brain injury or a stroke, with a special interest in the efficacy of blended cognitive behavioral therapy, combining face-to-face with online sessions. Based on our systematic review of merely small group studies, we concluded that CBT-I seemed effective for people with insomnia following traumatic brain injury or a stroke. Next, we developed a blended CBT-I intervention (eCBT-I). This intervention was based on standard CBT-I and adjusted to acquired brain injury. The efficacy of the newly developed eCBT-I was evaluated in a pilot SCED study and multicenter RCT. Both experimental studies showed that eCBT-I is feasible and effective with regard to clinical remission and in reducing insomnia severity posttreatment, and positive results remain at follow up compared to baseline. No significant improvement was found on fatigue, emotional well-being, cognitive functioning and societal participation. A cross-sectional study was conducted for a better understanding of the role that sleep-related beliefs and behaviors play in insomnia following acquired brain injury. Results showed that also in people with acquired brain injury, some sleep-related beliefs and behaviors are associated with insomnia severity, especially the beliefs regarding a diminished perception of control and worry about sleep. However, clinicians noted that several sleep-related beliefs and behaviors may not be considered dysfunctional, and may reflect adequate coping with the consequences of the brain injury. Findings of this thesis strengthens the body of evidence of efficacy of CBT-I for people with insomnia following a traumatic brain injury, also when delivered in a blended format. To translate findings of this thesis into clinical practice, a standard screening of insomnia in patients with acquired brain injury is suggested, as is the implementation of CBT-I as treatment of choice. The CBT-I could be delivered face-to-face, completely online or in a blended format. The thesis ends with guidelines how large scale implementation could be facilitated and suggestions for future research.