Three generations
In the first half of the 20th century the forgotten voice-type of countertenor was revived by pioneers like Russell Oberlin and Alfred Deller:
This encouraged a second generation to take up the art, which basically went hand in hand with the development of 'historically informed performance' practice of interpreting pre-classical era music. A good example is René Jacobs:
René Jacobs, in turn, taught Andreas Scholl, one of the very best currently active countertenors:
I personally love the countertenor voice (although there are still plenty of people who can't stand the somewhat strange timbre) and it's fascinating to hear how much the art has developed in the last 50 years.