When Pittsburgh's KDKA aired live returns from the presidential election race between Warren Harding and James Cox, it delivered the world's first commercial radio broadcast, according to the Federal Communications Commission, which regulates radio and TV in the U.S. Lasting 18 hours, from 6 p.m. on November 2 until noon the next day, the transmission was something of a makeshift affair. KDKA, owned by the Pittsburgh-based Westinghouse Corporation, had received its broadcast license just six days before the election. And the returns were beamed with a 100-watt transmitter from a small shed atop the highest building in Westinghouse’s East Pittsburgh plant.

While the broadcast reached only an estimated 1,000 listeners, it revolutionized how news could be delivered—as it happened in real time, instead of through newspapers printed and distributed hours or days later. And it inspired KDKA to more radio firsts: The following year, in August 1921, the station would air the first live, play-by-play broadcasts of a professional baseball game (Pittsburgh Pirates vs. Philadelphia Phillies) and two months later, a college football match (West Virginia University vs. University of Pittsburgh).

On the four-year anniversary of the election broadcast in 1924, The New York Times reported that KDKA’s 1920 broadcast experiment—enabled by the technological innovations of Guglielmo Marconi and Nikola Tesla—had started something big. Within a few years, more than 530 broadcasting stations had sprung up in the United States, reaching an estimated 10 million people.

In late November of 1925, in a new show called “Barn Dance” on Nashville's WSM radio station, announcer George D. "Judge" Hay introduced the program’s first-ever performer: octogenarian fiddler Jimmy Thompson playing foot-tapping, old-time music with his niece on piano accompaniment. Two years later, Hay changed the name of the show to the “Grand Ole Opry,” signaling in an on-air riff that they would offer a homier style of music than the grand opera show that preceded it on air each week.

Sears-Roebuck, a key KSM advertiser, initially opposed what it called the show’s “disgraceful low-brow music,” but fan letters poured in. By 1930, the Opry had 30 regular cast members, and WSM built a 500-seat studio theater to handle what was quickly becoming as much a live event as a radio broadcast. In 1932, WSM boosted its broadcast power to 50,000 watts, extending its reach to most of the U.S. and parts of Canada, and allowing country music to grow in popularity. The show introduced the world to bluegrass and country legends from Bill Monroe to Hank Williams to Dolly Parton.

“In a very real sense, the history of the Grand Ole Opry radio broadcast is the history of commercial country music,” wrote David Bruenger in Making Money, Making Music: History and Core Concepts. “Everything we know today about the Nashville sound, Nashville publishing, record labels and celebrities is the result of the opportunism and reach of this amazing show.”

Between March 1933 and June 1944, through Depression and war, President Franklin D. Roosevelt gave 30 speeches where he spoke directly to millions of Americans through a radio broadcast. These speeches became known as the "fireside chats,” a term coined by CBS station manager Harold Butcher because of President Roosevelt’s conversational speaking style. The first one began: “I want to talk for a few minutes with the people of the United States about banking.”

For their first fight on June 19, 1936, Black American boxer Joe Louis was a 10-to-1 favorite over Max Schmeling, but the German won the fight in a 12th-round knockout at Yankee Stadium. In the rematch two years later, Louis got his revenge with a technical knockout in the first round.

It's hard to underestimate the cultural impact of this sporting event. Fought against the backdrop of increasing Nazi aggression in Europe, the second fight is believed to have had the largest audience in history for a single radio broadcast—with an estimated 70 million listeners, according to the Library of Congress, which selected it in 2005 for its National Recording Registry.

As the Japanese attacked the U.S. naval base at Pearl Harbor on December 7, 1941, an unknown KTU reporter in Honolulu explained what was happening in real time:

The reporter, transmitting his report through phone lines to NBC in New York, was providing the nation with the only live broadcast of the surprise Pearl Harbor attack. At the time of the ferocious WWII air assault—which killed more than 2,400 Americans, damaged or destroyed nearly 20 naval vessels and more than 300 aircraft—there were 45 million radios in the United States. Around the nation, millions had their regularly scheduled programs interrupted by the historic news that the war had arrived on American shores.

On the night of October 30, 1938, between 8:15 and 9:30 p.m., a radio dramatization of H.G. Wells' sci-fi fantasy novel The War of the Worlds, performed by 23-year-old Orson Welles' Mercury Theatre, sent thousands of Americans into a frenzy. After hearing the broadcast, many believed that an interplanetary conflict had started with the invasion of Martians spreading death and destruction in New Jersey and New York.

The 1951 National League pennant game at New York's Polo Grounds between the Brooklyn Dodgers and the New York Giants may have been the first-ever nationally televised broadcast baseball game—but it’s best known for the radio broadcast. The dramatic moment came in the final game of a three-game, winner-take-all series. The Giants were down 4-2 in the 9th inning when Bobby Thompson came up to bat with runners on second and third base.

Russ Hodges' call on WMCA of Thompson's pennant-clinching three-run homer is one of the most memorable radio moments of the 20th century:

In 2020, Hodges’s iconic call of the Thomson home run was selected by the Library of Congress for inclusion in the National Recording Registry.

Between 1939 and 1941, CBS News Radio correspondent Edward R. Murrow's dramatic live reports from London during World War II made the horrors of war immediate and visceral. On September 21, 1940, as Nazi Germany bombed London, Murrow transmitted this gripping report from a rooftop:

For millions who followed the war in Europe from the safety of their homes an ocean away, Murrow's word pictures—accompanied by thudding bombs, snapping antiaircraft guns and shrieking whistles—helped generate support for the necessity of America's entering the war and aiding Allied forces against Nazi aggression.

“Murrow brought World War II into the living rooms of American homes,” wrote Bob Edwards, the former NPR reporter and author of Edward R. Murrow and the Birth of Broadcast Journalism. “Rarely had people heard the sounds of actual war unless they had fought in one themselves. To hear the shooting along with Murrow's outstanding reporting was something new and exciting. It established radio's place as a legitimate source of journalism.”