Clinical judgement is a crucial part of autism and attention deficit hyperactivity disorder (ADHD) assessments. This study aims to harness insights from psychology and sociology to explore clinical perspectives and assessment practices of autism and attention deficit hyperactivity disorder. Qualitative interviews were conducted with health care professionals (n = 17). Interviews were conducted between January and May 2019. Data were analysed using a thematic approach. Themes were sorted into two interrelated categories (1) approaches to diagnosis (2) elements of diagnosis. Approaches to diagnosis were comprised of the following subthemes: nature of diagnosis, application of diagnosis: natural and pragmatic, revisiting conceptualisations: sub-optimal practice & unhelpful diagnoses, autism and attention deficit hyperactivity disorder. Elements of diagnosis contained three related subthemes: contextualising standardised assessments, triangulating material, organisational factors. There is sometimes a pragmatic as well as nosological dimension to diagnosis. Competing desires for consistency and utility add further complexity to neurodevelopmental assessment.