The potential relationship between childhood obesity and earlier puberty onset has major public health implications. Earlier menarche in girls is associated with increased risk of adult obesity, type 2 diabetes and breast cancer. Current methods for assessing puberty are unreliable, with a lack of consensus regarding the impact of childhood obesity on breast development and/or age of menarche. Effects of obesity on early puberty in boys are more contentious, necessitating development of robust biomarkers. The possibility of the obesity epidemic lowering the age of puberty onset fuels concerns over the growing mismatch in age of sexual and social maturity. Here, we describe the biological basis linking childhood obesity to early puberty and consider evidence for a trend towards its earlier onset.