We explored the masculinity norms shaping transitions through puberty in rural and urban Tanzania and how these norms and their social-ecological context contribute to high-risk health behaviors. We conducted a qualitative case study of adolescent boys in and out of school in 2011 and 2012. Tanzania's social and economic development is reshaping the transition into young manhood. Adolescent boys are losing traditional mechanisms of pubertal guidance, and new meanings of manhood are arising from globalization. Traditional masculinity norms, including pressures to demonstrate virility and fertility, remain strong. Adolescent boys in modernizing Tanzania receive inadequate guidance on their burgeoning sexuality. Contradictory masculinity norms from family and society are shaping their sexual expectations, with implications for their engagement in unsafe sexual behaviors.