The molecular bases of behavior problems in young infants are related to genetic and epigenetic determinants of children's behavior. It has been estimated that 7% of preschoolers and 13-20% of school-aged children and adolescents suffer from internalizing and/or externalizing problems. Researchers in the field are involved in the search for specific epigenetic patterns related to the prenatal environment using cutting-edge approaches like the candidate gene, the genome-wide, and epigenome-wide approaches. The present review article will discuss all these latest innovations in order to better understand molecular bases of behavior problems in young infants that shall definitely result in better treatment planning.