This paper considers diagnostic frameworks in developmental–behavioural paediatrics. The purpose of a diagnostic assessment is reviewed, and the use of categorical diagnoses is explored. A multi-level process of diagnostic formulation is outlined, highlighting the importance of a comprehensive focus on presenting symptoms, neuropsychological constructs, biological factors and environmental influences. The axis of time, developmental benchmarks and the enhancement of resilience are discussed as part of the diagnostic formulation framework. Limitations imposed on diagnostic practice by systemic and personal factors are reviewed. Implications for training and practice are discussed.