Attention deficit - hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder of childhood and comprises a range of behavioral problems, including inattention, impulsivity and hyperactivity. ADHD has been investigated extensively during the last 30 years. At this point most researchers agree that ADHD is a problem of complex etiology that needs to be investigated as a function of complex interactions. Autism Spectrum Disorders (ASD) is a chronic disorder, with onset before three years of age. It is one of the fastest growing developmental disabilities in the United States and Puerto Rico. It presents with a wide range of stereotyped, repetitive behaviors, social and language impairment. Function and outcome is affected not only by core deficits but by associated behaviors such as hyperactivity, aggression, anxiety, and depression. Many studies have indicated that behavioral therapy and medication may be at least partially helpful in the treatment of children with ADHD or with ASD. Research on the effect of diet and nutrition on ADHD and autism has been increasing in the past two decades, particularly on the symptoms of hyperactivity and attention. Particular attention has focused on the role of food additives, refined sugar, food allergies, and fatty acid metabolism. It is imperative that data supporting new treatments should be scrutinized for scientific study design, clinical safety, and scientific validity, before embarking on them as modes of therapy. This updated review presents the evidence regarding the usefulness and limitations of the most frequent nutritional and dietary interventions in the treatment of ADHD and/or ASD.