Abstract Attention-deficit/hyperactivity disorder (ADHD) and autism are neurodevelopmental disorders that emerge in childhood. There is increasing recognition that ADHD and autism frequently co-occur. Yet, questions remain among clinicians regarding the best ways to evaluate and treat co-occurring autism and ADHD. This review outlines issues relevant to providing evidence-based practice to individuals and families who may be experiencing difficulties associated with co-occurring autism and ADHD. After describing the complexities of the co-occurrence of autism and ADHD, we present practical considerations for best practice assessment and treatment of co-occurring autism and ADHD. Regarding assessment, this includes considerations for interviewing parents/caregivers and youth, using validated parent and teacher rating scales, conducting cognitive assessments, and conducting behavior observations. Regarding treatment, consideration is given to behavioral management, school-based interventions, social skills development, and the use of medications. Throughout, we note the quality of evidence that supports a particular component of assessment or treatment, highlighting when evidence is most relevant to those with co-occurring autism and ADHD across stages of development. In light of the current evidence for assessment and treatment of co-occurring autism and ADHD, we conclude by outlining practical implications for clinical and educational practice.