Leung Chun-ying is the leader of Hong Kong. As chief executive, he signs bills into law, issues executive orders, appoints and removes judges and other public officials, and pardons convicted criminals. He’s the leader—except that last fall well over 100,000 Hong Kongers chose dramatically not to follow him. When they learned that the 2017 election for Leung’s position would not be free and democratic, as authorities had previously suggested, they poured into the streets and followed Joshua Wong, then 17, who had started a pro-democracy student group. Leung, 60, commanded a vast city administration, including police wielding pepper spray and truncheons. Wong had a cellphone. Yet the protesters paralyzed Hong Kong for three months, Leung’s already low approval ratings plunged to their lowest ever, and Wong landed on the cover of Time’s Asia edition, which called him the “Voice of a Generation.”

So who’s the real leader? The answer is obvious: Leung has the leader’s job, but he doesn’t have leadership. Wong is the one who demonstrated that—which is why he’s the one on our 2015 roster of the World’s 50 Greatest Leaders.

We cast a wide net in assembling our list, which includes leaders without any formal designation, like Wong, as well as elected government officials, CEOs, chiefs of NGOs, clergy, coaches, athletes, artists, and more—all judged on their leadership within their professional domains, industries, or fields of service or governance. To make this roster, it was not enough to be brilliant, admirable, or even supremely powerful (see Moisés Naím’s essay on Vladimir Putin). We set out to find singular leaders with vision who moved others to act as well, and who brought their followers with them on a shared quest. We looked for effectiveness and commitment and for the courage to pioneer. All had to be active in leadership roles, though a long history of leading is something that many on our list share. And only a few are repeats from last year; in each case, he or she had to requalify with new achievements in the past 12 months. We sought nominations for this year’s group from those on last year’s list and from a wide range of leadership experts, then added names turned up by Fortune reporters. We vetted our nominees with appropriate experts and made our judgments.

The generational difference between Leung and Wong symbolizes a larger point: that today’s leadership isn’t the same as yesterday’s. Yes, some elements are eternal, and it’s tempting to believe we can learn what we need to know by studying, as various books have urged, the leadership secrets of Genghis Khan, Ulysses S. Grant, the Salvation Army, and Santa Claus, among others. We can learn plenty from them, but in truth, effective leadership is forever changing, and like everything else, it’s changing faster than ever. Successful leadership may be eternal, but it’s also brand-new.

The most important change affecting today’s leaders is that they’re losing control. Whether they’re leading a company, a sports team, a nation, a congregation, the supporters of an idea, or any other group, their success depends increasingly on influencing people they cannot control with money or force. Lest you think it was ever thus, note what Robert Malott, CEO of chemical company FMC in the 1970s and 1980s, once said to Fortune: “Leadership is demonstrated when the ability to inflict pain is confirmed.” A lot of CEOs really liked that quote back then, but today if you asked 10 friends who said it, I suspect most of them would guess Vladimir Putin. (He isn’t on our list.)

Today’s starkly different reality is why the Harvard University Initiative for Advanced Leadership exists. “The premise behind our initiative is that the problems you have to deal with now are messy, complex problems outside your organization,” says the program’s director, Harvard Business School professor Rosabeth Moss Kanter. “You can’t tell people what to do. You have to deal with many constituencies that you don’t control.”

A good example is the massive recall mess that Mary Barra faced upon becoming General Motors CEO in January 2014. Her job ultimately is to lead 212,000 employees in a way that maximizes returns to GM’s owners, but to do it she has had to, among many other tasks, work skillfully with victims’ families, federal regulators, U.S. senators, the Justice Department, at least one federal judge, plaintiffs lawyers, and media. Oh, and now an activist shareholder, Harry Wilson, representing four hedge funds: Together they own just 2.1% of GM, yet Wilson’s call for the company to give more cash back to shareholders attracted global attention. After negotiations, Barra agreed to return $5 billion to shareholders—while also reassuring employees, investors, and suppliers that even without that money to invest, GM would still “deliver sustained profitable growth.” Barra’s crucial constituencies are almost beyond counting.

Or consider an even more extreme example, the situation of Joanne Liu, international president of Médecins Sans Frontières (Doctors Without Borders). She controls practically no one. The organization’s office staff is modestly paid, but the tens of thousands of doctors, nurses, and others who deliver relief in many of the world’s most dangerous places are volunteers. Governments can keep them out. Local people sometimes attack or kidnap them. Only a vast, easily disrupted network of voluntary cooperation enables anything good to happen—and yet it happens.

Even employees can’t be controlled the way they used to be. “As an employer, you used to look at the workforce as your workforce—they belonged to you,” says Josh Bersin, a consultant on human capital strategies. “Now, because of changes in employee values, you have to think of your workforce as a bunch of volunteers.” The best ones, especially among millennials, want to work for organizations pursuing a worthy mission. And through social media and employment websites, they’re continually aware of other jobs available to them and how other workplaces stack up against their own. It’s no use trying to push them around.

As a handy rule of thumb, leaders today may just want to assume that everyone knows everything, and everyone can communicate with everyone else at any time. Those assumptions aren’t strictly true, of course, but they’re close enough to be useful. Doomed leaders see the new reality as a damnable constraint on their effectiveness; great leaders see it as a huge opportunity. They can gather more information more easily than their predecessors would have dreamed possible, and they can communicate with their many constituencies in more ways and more often. Even with less control, a leader’s opportunities to be effective are greater than ever.

Disoriented by the new rules, leaders can find comfort in the eternals. People everywhere still want leaders who will speak to them honestly about the challenges they face together. They want leaders who show courage and see hope, who say, “I see the way to a better future. Follow me.” That was the message that brought the two mayors on our list, Mike Duggan of Detroit and Tri Rismaharini of Surabaya, Indonesia, to the helm of the troubled cities they’re turning around.

Most deeply, people still want to be led. They understand instinctively that no group achieves anything worthwhile without someone in charge. That’s why, especially in crises, people inevitably rally around a leader. What’s strikingly new is that in a radically more open, more connected world, that leader could be the designated authority—or it could be a 17-year-old kid with a cellphone. Our World’s Greatest Leaders are the best examples we’ve found of how to succeed in this challenging new environment, where leaders must earn their leadership every day.

How Fortune built this year’s list: In compiling the 2015 list of the World’s 50 Greatest Leaders, we gathered advice, fittingly, from more than two dozen of the world’s best minds. Their suggestions were invaluable, but the editors of Fortune accept full responsibility for the final list. Here is our full list of nominators: Mildred Apenyo, Dominic Barton, Kathy Bloomgarden, Ian Bremmer, Kenneth Chenault, Caitlin Colegrove, Sen. Christopher Coons, Rohitesh Dhawan, Susan Desmond-Hellmann, Charlotte Florance, Laurie Garrett, Ilene Gordon, Karl-Theodor zu Guttenberg, Richard Haass, Rosabeth Moss Kanter, Thomas Kolditz, Anand Mahindra, Rita Gunther McGrath, Denise Morrison, Alan Mulally, Joseph Nye, Jeffrey Pfeffer, Zhang Ruimin, Carol Sawdye, Witney Schneidman, Peter Thiel, Paul Volcker, and the Fortune staff.

This story is from the April 1, 2015 issue of Fortune.