This study by W. A. Marshall and J. M. Tanner examines the variations in the pattern of pubertal changes in boys, based on longitudinal data from 228 normal boys. The genitalia began to develop between 9.5 and 13.5 years of age in 95% of boys, with a mean age of 11.6 ± 0.99 years, and reached maturity between 13 and 17 years, with a mean age of 14.9 ± 1.10 years. Pubic hair development reached an adult female distribution at a mean age of 15.2 ± 0.01 years. The genitalia reached the adult stage 3.0 years after they first began to develop, with some boys completing this process in as little as 1.8 years and others taking up to 4.7 years. The peak height velocity (PHV) was reached at a mean age of 14.1 ± 0.14 years, with 41 boys reaching it at a mean age of 14.1 ± 0.14 years. The study also found that boys tend to reach PHV at a later stage in the development of their genitalia compared to girls, with only 2% of boys reaching PHV before genital Stage 4, compared to 76% of girls reaching PHV before breast Stage 4. The data provide a basis for distinguishing normal from abnormal puberty in boys, suggesting that genital development is normal if it begins after 9 years and ends before 15 years.This study by W. A. Marshall and J. M. Tanner examines the variations in the pattern of pubertal changes in boys, based on longitudinal data from 228 normal boys. The genitalia began to develop between 9.5 and 13.5 years of age in 95% of boys, with a mean age of 11.6 ± 0.99 years, and reached maturity between 13 and 17 years, with a mean age of 14.9 ± 1.10 years. Pubic hair development reached an adult female distribution at a mean age of 15.2 ± 0.01 years. The genitalia reached the adult stage 3.0 years after they first began to develop, with some boys completing this process in as little as 1.8 years and others taking up to 4.7 years. The peak height velocity (PHV) was reached at a mean age of 14.1 ± 0.14 years, with 41 boys reaching it at a mean age of 14.1 ± 0.14 years. The study also found that boys tend to reach PHV at a later stage in the development of their genitalia compared to girls, with only 2% of boys reaching PHV before genital Stage 4, compared to 76% of girls reaching PHV before breast Stage 4. The data provide a basis for distinguishing normal from abnormal puberty in boys, suggesting that genital development is normal if it begins after 9 years and ends before 15 years.