Majority of the brain growth and development occurs during the first 1000 days of life i.e. from conception till two years. This period is known as the critical period. Animal and human studies suggest that under nutrition during this critical period can adversely affect brain growth and development. Human studies carried out among preterm/full-term infants born with IUGR/VLBW/SGA reveal that early under nutrition may impact key cognitive functions such as – attention, memory, visuo-motor, visuospatial and executive functions. Nutritional supplementation during the critical period along with psychological stimulation may aid in preventing cognitive deficits in undernourished children and those at-risk.