Logistic curves have been fitted to the growth during puberty of the 55 boys and 35 girls of the Harpenden Growth Study who were measured every three months during puberty and thereafter until growth ceased. Very good fits were obtained for stature, sitting height, subischial leg length, biacromial and bi-iliac diameters from approximately six months after the beginning of the adolescent spurt. This beginning, called "take-off", was determined graphically as the point of minimum velocity. The total height gained from take-off point to cessation of growth averaged 28 cm in boys and 25 cm in girls with standard deviations of about 4 cm. The adult sex difference in height was due much more to the later take-off in boys than to a greater male adolescent spurt. A sex difference in the spurt occurred in sitting height but not in leg length. Mean-constant curves for the four measurements are presented. In each measurement size at take-off and total adolescent gain were nearly independent, the average correlation coefficient being --0-2. The correlations between adolescent gains in different measurements averaged only 0-47, and between peak velocities of different measurements only 0-27. This implies considerable shape change at adolescence. In contrast the average correlation between ages at which the peak velocities were reached was 0-87. Ages at take-off, at peak velocity, and at menarche were independent of mature size, though correlated with percentage of adult size reached at the ages in question, a measure of somatic maturity. Relationships with the development of breasts, pubic hair and genitalia were examined; ages at take-off and at peak velocity correlated to the extent of 0-6 to 0-8 with ages of B2 and PH2 but both these parameters and also peak velocities were uncorrelated with the rapidity with which sex characters developed.