While specific numbers may be disputed, most sources agree that Islam is the fastest growing religion in the United States, in France, and worldwide. Growth rates and future projections are based on combinations of current Muslim population counts, fertility and religious conversion rates, and immigration data. These projections are sometimes labeled as �scenarios� implying a high degree of imprecision and dependence upon initial assumptions. The current global Muslim population of around 20% is likely to rise, by the most conservative estimates, to about 24-25% of the world�s total by the year 2025. Other sources — many of them Muslim — predict future percentages closer to 30%, which would have Islam surpass Christianity as the most populous faith community worldwide. As with population counts, these reported growth rates are likely to be influenced by religious and political agendas.

The factors related to Muslim population growth vary over time and changing circumstances. It is frequently reported that birth rates among Muslim women, particularly recent immigrants, tend to be higher than average levels in Western nations. However, these rates tend to fall, particularly as immigrant groups become established and incomes rise. Immigration policies and patterns have also changed, particularly since September 11, 2001. This variability makes past national growth rates only an imprecise guide to future growth of Muslim populations. Some have chosen the number of mosques as a key indicator of the rise of Islam in Western countries. Though this is a helpful guide, many note that the sites counted are often small and informal prayer rooms, in basements and other places, rather than full and formal religious institutions.

The concerns related to the growth of Muslim populations differ from country to country. In France, a crucial factor is the concentration of Muslim populations in urban areas, where predictions are that as much as one-third of the next generation will be Muslim and some have warned of the formation of “ethnic ghettoes.” An important question for many in the American political scene is the relative size and rate of growth in Muslim and Jewish populations. The two groups make competing claims, and while it is likely that they are quite close in population, the Muslim community may now be slightly larger.