The New York Times makes the following assertion today:
The Brotherhood, founded in Egypt in 1928, is the oldest and largest Islamist movement in the world, with affiliates in nearly every Muslim country and adherents in Europe and the United States. Its size and diversity, and the decades-old legal ban that has kept it from genuine political power in Egypt, make it difficult to sum up simply. As the Roman Catholic Church encompasses leftist liberation theology and conservative anti-abortion advocacy, so the Brotherhood includes both practical reformers and firebrand ideologues.
For something more accurate, here.