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This study aimed to (1) identify a stable, trait‐like component to cortisol and its circadian rhythm, and (2) investigate individual differences in developmental trajectories of HPA‐axis maturation. Multiple salivary cortisol samples were collected longitudinally across four assessments from age 9 (3rd grade) through age 15 (9th grade) in a community sample of children (N = 357). Sophisticated statistical models examined cortisol levels and its rhythm over time; effects of age, puberty and gender were primarily considered. In addition to situation‐specific and stable short‐term or epoch‐specific cortisol components, there is a stable, trait‐like component of cortisol levels and circadian rhythm across multiple years covering the transition from childhood into adolescence. Youth had higher cortisol and flatter circadian rhythms as they got older and more physically developed. Girls had higher cortisol, stronger circadian …