Anxiety disorders (ANX) and attention-deficit/hyperactivity disorder (ADHD) commonly co-occur; this comorbidity is associated with a high degree of impairment and a poor long-term prognosis. In this review, the nature of this comorbidity and its treatment will be described, and an approach to treating children with ANX + ADHD will be presented. The etiology of ANX + ADHD is controversial, with biological, developmental, environmental and cognitive factors examined in various studies. There is increasing evidence that ANX + ADHD may be distinct from each separate disorder, and may represent a neuropsychiatric condition that involves dysregulation in both anxiety and ADHD domains. Treatment usually requires a combination of medication and psychotherapeutic intervention. Stimulant medications have been found most helpful so far compared with other medications, although atomoxetine is also being studied. There is limited evidence for selective serotonin reuptake inhibitors, and their potential for behavioral activation may be problematic in these children. Intensive behavior modification was shown to be beneficial in conjunction with medication for ANX + ADHD in a multimodal treatment study of children with ADHD. Cognitive–behavioral therapy has been used to address anxiety symptoms, but may need to be individualized in ANX + ADHD as cognitive limitations and ADHD behaviors may otherwise interfere. Parental anxious or ADHD traits and the child’s developmental level must also be considered to optimize treatment.