What is it like to teach a free online course to tens of thousands of students? Dozens of professors are doing just that, experimenting with a format known as Massive Open Online Courses. And there are more providers than ever, some working with elite universities, and others that allow any professor to join in.

The Chronicle asked four professors, teaching on different platforms, to share their thoughts on the experience so far. The responses are based on e-mail interviews, which have been condensed and edited for publication.