“I cannot think of a better time in our nation’s history for the brave names of Alicia Garza, Patrisse Cullors, and Opal Tometi to be recognized. The courage heard in their voices mirrors some of history’s greatest giants, with sharp echoes of Rosa Parks. Black Lives Matter is working to heal our country’s age-long sickness. For when one is sick, so too are we all sick. Only when all is healed might we all one day be well.” —Uzo Aduba, actress (Read Uzo Aduba’s comments in full at the end of this piece.)

They were always worried about their brothers.

Patrisse Cullors was 13 when she watched Los Angeles police handcuff and haul away her older brother without knowing why it was happening.

Growing up in a Phoenix suburb, Opal Tometi, the daughter of Nigerian immigrants, was alarmed when her youngest brother started preschool and began to raise questions about his hair and skin color—questions she knew were triggered by societal messages about race.

And Alicia Garza worried about her brother’s safety every day—but never so much as after July 13, 2013, when George Zimmerman, a neighborhood watch volunteer in Florida, was found not guilty in the shooting death of 17-year-old Trayvon Martin. “My brother is six feet tall and has a huge Afro,” Garza says, “and I thought, That could have been my family.”

The night of the acquittal, all three women were devastated. But as they mourned, they turned their sorrow and outrage into action, creating a powerful civil rights movement that, in just three years, has transformed the way Americans think and talk about race. Garza and Cullors had met at a conference for activists nearly a decade earlier. (“We just fell in love instantly,” recalls Garza. “We call each other ‘Twin.’”) The night of the verdict, they texted, sharing their grief. “When I woke up in the morning,” says Garza, 35, who is the special projects director of the National Domestic Workers Alliance in Oakland, California, “I wrote a love letter to black people.” Her now-famous Facebook posts are a lament, an exhortation, and a praise song. “I continue to be surprised at how little black lives matter,” she wrote. She ended with, “Black people. I love you. I love us. Our lives matter.”

Cullors, 33, a Los Angeles–based organizer and artist, shared the posts on Facebook, spontaneously finishing her own post with #BlackLivesMatter. Tometi, 32, the executive director of the Black Alliance for Just Immigration in New York City, saw the hashtag and reached out to Garza, whom she knew from the activist community, and volunteered to build a digital platform.

“I felt a sense of urgency about the next steps we could take together to change the story,” Tometi says.

Adds Garza: “We wanted to connect people who were already buzzing about all this stuff and get them to do something, not just retweet or like or share. We thought, How do we get folks together and take that energy and create something awesome?”

With that, #BlackLivesMatter—a rallying cry for a new generation—was born.

Given the facts of American history, it was all too predictable that Martin’s would not be the last widely reported killing of an unarmed black person. And when a new case hit the headlines—the August 2014 death of Michael Brown, 18, in Ferguson, Missouri, at the hands of a white police officer—Black Lives Matter roared into action to create a “freedom ride,” so protesters from around the country could get to Ferguson. The three women also made a key decision: To keep their group decentralized. Today the Black Lives Matter Global Network is a coalition of 42 autonomous chapters, each doing its own work. The Chicago chapter, for example, helped oust the police superintendent after video footage of an officer shooting a black teenager was withheld for more than a year. In addition to protesting racism and unlawful killings, Black Lives Matter groups have taken on inequality, sexism, homophobia, and transphobia.