Background: Adolescence is a time of considerable social, cognitive, and physiological development. It reflects a period of heightened risk for the onset of mental health problems, as well as heightened opportunity for flourishing and resilience. The CogBIAS Longitudinal Study (CogBIAS-L-S) aims to investigate psychological development during adolescence. Methods: We present the cohort profile of the sample (N = 504) across three waves of data collection, when participants were approximately 13, 14.5, and 16 years of age. Further, we present descriptive statistics for all of the psychological variables assessed including (a) the self-report mood measures, (b) the other self-report measures, and © the behavioural measures. Differential and normative stability were investigated for each variable, in order to assess (i) measurement reliability (internal consistency), (ii) the stability of individual differences (intra-class correlations), and (iii) whether any adolescent-typical developmental changes occurred (multilevel growth curve models). Results: Measurement reliability was good for the self-report measures (> .70), but lower for the behavioural measures (between .00 and .78). Differential stability was substantial, as individual differences were largely maintained across waves. Although, stability was lower for the behavioural measures. Some adolescent-typical normative changes were observed, reflected by (i) worsening mood, (ii) increasing impulsivity, and (iii) improvements in executive functions.Conclusions: The stability of individual differences was substantial across most variables, supporting classical test theory. Some normative changes were observed that reflected adolescent-typical development. Although, normative changes were relatively small compared to the stability of individual differences. The development of stable psychological characteristics during this period highlights a potential intervention window in early adolescence.