Objectives: Educational practitioners play an important role in the referral and treatment of children with attention-deficit/hyperactivity disorder (ADHD). This study aimed to explore how educational practitioners conceptualise their beliefs about the causes of symptoms of ADHD. Method: Forty-one educational practitioners from schools in the United Kingdom participated in focus groups or individual interviews. Data were analysed using thematic analysis. Results: Practitioners' beliefs fell into two categories: biological and environmental. Practitioners conceptualised the causes of ADHD in lay-theoretical models: a "True" ADHD model considered that symptoms of ADHD in many cases were due to adverse environments; and a model whereby a biological predisposition is the root of the cause of the child's symptoms. Conclusion: Differential beliefs about the causes of ADHD may lead to practitioners blaming parents for a child's behaviour and discounting ADHD as a valid condition. This has implications for the effective support of children with ADHD in schools.