Research indicates that adolescents are at risk for insomnia, but are reluctant to seek help. Treatment of insomnia has been extensively examined in adults, but studies with adolescents are sparse. The purpose of this pilot study was to assess feasibility and efficacy of cognitive behavioral therapy for insomnia (CBT-i) for adolescents in both group and Internet settings. Twenty-six adolescents received 6 weekly sessions of CBT-i in a group (N = 13) or individual Internet setting (N = 13). Their sleep was measured with actigraphy, sleep logs, and questionnaires at baseline, posttreatment, and a 2-month follow up. For both treatments, results show a significant improvement, with medium to large effect sizes (ESs) of sleep onset latency, wake after sleep onset, and sleep efficiency. There was also a small ES increase of total sleep time in sleep log measures, but not in actigraphy measures. On questionnaires measuring symptoms of insomnia and chronic sleep reduction, significant improvements occurred either at posttreatment or at follow up. No differences were found between the groups. This study indicates CBT-i, either in group or in Internet formats, is an effective treatment for insomnia in adolescents. Further studies in a randomized controlled design are warranted.