This paper aims to study the challenges and the opportunities due to the demographic transition emphasizing on Iran's past and the probable future experiences. The first demographic transition that concurrently accompany with modernization and development processes, create some challenges such as increasingly high population growth rate and expansion of population size, young age structure and increase in the population at work ages. The second demographic transition begins when societies enter post-modernization stage. The most important challenges of second demographic transition include the below replacement fertility, reduction of population at work age, population ageing and the probable negative growth rate of population. Of course, regarding particular conditions of societies, the pace of changes maybe quite different. Iran has experienced the challenges of the first demographic transition during the past half a century. If approach of tendentious regularity of social changes is implicitly accepted, we should expect the challenges of the second demographic transition in Iran at the forthcoming decades. Population planning and proper population policies can give a smooth path to the future challenges of the second demographic transition in Iran.