Vitamin B12 plays a vital role in normal brain functioning. Its deficiency is associated with developmental delays, irritability, and failure to thrive in children. Several studies have evaluated the possibility of B-12 deficiency leading to ASD/ADHD with inconsistent results. This review summates the available literature on B12 deficiency/supplementation and ASD/ADHD in children. The literature search was conducted in Medline and Extended Medline using the following search words (Vitamin B12 or Cobalamin or Methylcobalamin or Cyanocobalamin) and (Autism, Autistic spectrum disorder or attention deficit hyperactivity disorder or ASD or ADHD). Total of sixteen studies were retrieved of which four were clinical trials. Most of the observational studies report an association between B-12 deficiency and Autism. Studies on ADHD are fewer, and the association is weaker. Intervention studies were only available for ASD. Three of these reported improvement in biochemical and/or clinical behavior ratings for ASD children while one study did not show any improvement in either. There is observational data documenting the relationship between B-12 dfefficiency and ASD. Studies on ADHD are fewer and less conclusive. Intervention studies using B-12 in ASD children document biochemical improvement. There is paucity of trial literature on the clinical impact of B-12 supplementation in ASD especially in terms of behavior ratings. Further work should be urgently considered to address these lacunae.