Teaching Introduction to Sociology is almost second nature to Princeton professor Mitchell Duneier: He has taught it 30 times, and a textbook he co-wrote is in its eighth edition.

But last summer, as he transformed the class into a free online course, he had to grapple with some brand-new questions: Where should he focus his gaze while a camera recorded the lectures? How could the 40,000 students who enrolled online share their ideas? And how would he know what they were learning?

In many ways, the arc of Mr. Duneier's evolution, from professor in a lecture hall to online instructor of tens of thousands, reflects a larger movement, one with the potential to transform higher education. Already, a handful of companies are offering elite college-level instruction -- once available only to a select few, on campus, at great cost -- free, to anyone with an Internet connection.

Moreover, these massive open online courses, or MOOCs, harness the power of their huge enrollments to teach in new ways, applying crowd-sourcing technology to discussion forums and grading, and enabling professors to use online lectures and reserve on-campus class time for interaction with students.

The spread of MOOCs is likely to have wide fallout. Lower-tier colleges, already facing resistance over high tuition, may have trouble convincing students that their courses are worth the price. And some experts voice reservations about how online learning can be assessed, and warn of the potential for cheating.

MOOCs first hit the spotlight last year, when Stanford professor Sebastian Thrun offered a free Artificial Intelligence course, attracting 160,000 students in 190 nations. The resulting storm of publicity galvanized elite research universities across the nation to begin to open higher education to everyone -- with the hope of perhaps, eventually, making money doing so.

The expansion has been dizzying. Millions of students are now enrolled in hundreds of online courses, including those offered by Udacity, Mr. Thrun's spinoff company; edX, a joint Harvard-MIT venture; and Coursera, a Stanford spinoff offering Mr. Duneier's course and 200 others.

Top universities with courses such as Mr. Duneier's stand to gain in prestige. The risks are greater for lesser colleges, which may be tempted to drop some introductory courses -- and professors who teach them -- and substitute cheaper online instruction from big-name professors.

"We've reached the tipping point, where every major university is thinking about what they will do online," said Peter McPherson, president of the Association of Public and Land-Grant Universities. "In a way, the most important thing about these MOOCs from the top universities is that they provide cover, so other universities don't need to apologize about putting courses online."

Elite universities are lining up to join forces with a MOOC provider. Coursera, begun in April with Princeton, Penn, Stanford and the University of Michigan, leads the field, with 33 university partners. But edX, too, is expanding rapidly -- the University of California, Berkeley has joined, and the University of Texas announced it will use edX courses, for credit. Already, students in one Udacity class can get credit through the Colorado State University Global Campus.

Most MOOC providers are making plans to offer credit -- and charge fees for certificates and proctored exams.

Mr. Duneier has been thrilled. "Within three weeks, I had more feedback on my sociological ideas than I'd had in my whole teaching career," he said. "I found that there's no topic so sensitive that it can't be discussed, civilly, in an international community."

The online discussion forum spawned many global exchanges. Soon after Mr. Duneier talked about social norms, using as his example the lack of public restrooms for street vendors -- including an embedded video of New York vendors talking about the problem -- students in Hong Kong, India, Russia and elsewhere commented on the situation in their own cities.

Meanwhile, around the world, study groups were forming. In Katmandu, Nepal, Dipendra K.C., 22, connected with four older classmates, meeting in person to prepare for the midterm and final. "We were looking at the lectures and the discussion forum and pointing out topics the professor was highlighting, to try to predict the questions on the exam," he said.

To create a Princeton seminar's feel, Mr. Duneier used a video chat room in which six or eight students -- Dipendra was one, others were from Siberia or Iran as well as Princeton -- discussed the readings; students over the course of the week could replay the video and comment.

As with other MOOCs, less than 5 percent of those who enrolled in the sociology course finished it: 2,200 midterm exams and 1,283 final exams were submitted. Some students listened to all the lectures and did all the readings but didn't take exams. There was no reason to do so since Princeton -- unlike Udacity, edX or other universities working with Coursera -- does not give certificates of completion.