"Demographic WINDOW" refers to a particular period, in the demographic history of a population, in which age-dependency ratio falls to an unprecedented low level. This period is short, the beginning and end of it can be estimated, and it lasts about three to four decades.The Demographic WINDOW of Iran began at the threshold of 2006 census and is expected to end by 2046. Therefore, it is expected to last for almost 4 decades. Thereafter, the population of Iran moves towards becoming old, to an unprecedented level in its history.In the demographic history of each country, demographic WINDOW opens once, and during the time it is open, it provides an exceptional opportunity for the development of the country. However, it is not automatic. It should be activated and guided. Demographic WINDOW becomes a demographic bonus only with appropriate preparation: policy-making, planning and correct implementation of programs.Several years, apparently unnoticed, passed since Iran entered its demographic WINDOW. Time is moving fast and this short time span becomes shorter. Therefore, it seems urgent to conduct required studies for the appropriate utilization of this unrepeatable opportunity for the development of the country.