The main purpose of this study is to assess the long-term safety and efficacy of lebrikizumab in participants 6 Months to \<18 years of age with moderate-to-severe atopic dermatitis. This study will last about 68 weeks and may include up to 16 planned visits. If participating in the optional extension as well, the study will last approximately 1 additional year for a total of up to 116 weeks and may include up to 29 scheduled visits.