When were women first welcomed into Buddhist orders?

I know that in the early days of Buddhism, it was for men only. To practice, you had to be a monk, and to be a monk, you had to be a man. Perhaps this reflects the attitudes of the time, but today, my experience of Buddhism has been as something that welcomes men and women equally, even when that's still in a segregated way. Historically, when did this change come about, and what first triggered it?