It's hard to define music by decade, because certain eras pop up inside some of those neatly derived constraints, so I'll define it by themes.

1954-1958: The birth of rock and roll (a.k.a, the Devil's music). Rock and Roll is born thanks to the rise of pioneering artists like Bill Haley, Chuck Berry, Little Richard, Jerry Lee Lewis, Buddy Holly, and of course, Elvis Presley. Despite mainstream cultural leaders decrying the music as a force of self-destruction and evil, youth loved it, and much of it was quite inncocent and pursued themes of love and fun. However, that innocence was shattered after the deaths of Buddy Holly, Ritchie Valens, and the Big Bopper in a tragic plane crash on a snowy and icy night, forever known as "the day the music died."

1959-1963: The mainstreaming of rock and roll and the rise of doo wop. By 1959 it became obvious that rock and roll was not a fad, so the earliest record producers rushed to take advantage of this fledgling music genre. The most obvious example was Colonel Tom Parker and his work with Elvis Presley, but a young Phil Spector created his own musical groups, and doo wop reigned on the charts. It started with African American artists but branched out to white performers and even spread to the west coast in the form of surf music by the Beach Boys and Jan and Dean.

1964-1966: The British Invasion and Motown: By the beginning of 1964, a whole new wave of young people were coming of age, and the generation known as the Baby Boom fell wildly in love with new artists coming from the UK and the Motor City. The Beatles, The Rolling Stones, The Who, The Kinks, Herman's Hermits, and the Dave Clark Five stormed onto the pop charts. Meanwhile, an assembly line auto worker from Detroit wanted to pursue a career as a music mogul, and he formed his own record company which would become Motown Records. His label launched the careers of The Supremes, The Temptations, The Four Tops, Marvin Gaye, Stevie Wonder, and the Jackson Five.

1967-1970: The counterculture revolution. By 1966 the Beach Boys were beginning to experiment with expanding the scope of rock and roll with the release of Pet Sounds, but it was the Beatles' release of Sgt. Pepper that would expand the genre and change rock and roll forever. Meanwhile, Bob Dylan picked up an electric guitar and changed folk music into something much meatier, and the likes of the Byrds, Jefferson Airplane, Jimi Hendrix, and The Doors came into prominence. This all dovetailed with the protests over the civil rights movement, the Vietnam War, and the nascent women's and LGBT movements. This musical expermentation grew into a burining fire that flamed out with the breakup of the Beatles and the deaths of Janis Joplin, Jimi Hendrix, and Jim Morrison of the Doors.

1971-1976: The rise of progessive rock, metal, soul, funk, and the singer-songwriter. After the turmoil of the late 60's and the losses of the Beatles and other artists, rock and roll took wildly divergent paths, which ranged from the experimental yet heavy sounds of Jethro Tull, Emerson, Lake, and Palmer, and Yes, to the harder-edged sounds of Led Zeppelin, Black Sabbath, and Thin Lizzy, to southern rock from the likes of the Allman Brothers and Lynyrd Skynard, to more introspective music from singer-songwriters like Elton John, Billy Joel, James Taylor, Carly Simon, and Carole King. Bruce Springsteen became the standard for working man's rock and roll. Meanwhile, R&B music became more soulful thanks to the likes of soul groups like the Spinners, the Whispers, the Delfonics, the Chi-Lites, and the Stylistics, while funkier groups with huge horn sections rose up thanks to Parlaiment-Funkadelic, the Ohio Players, and Kool & the Gang.

1977-1982: Disco, punk, new wave and the birth of hip-hop. While most of the country was thrilled with the artists of the early and mid-1970's, a couple of countercultures were rising up, primarily in New York but also across the pond and in Miami. A group of disaffected young people that the mainstream had wrtitten off as punks were creatying their own music which was faster, ediger, and more nihilistic than the rock that proceeded it. They were led in the USA by the Ramones and in the UK by the Sex Pistols. Though the Sex Pistols would tragically flame out in the USA, their spirit caught on in the UK and was forwarded by the Clash and refined by other artists with synth pop melodies from the likes of M and Joy Division. This would lead to the second British Invasion, or as it was more commonly called, "New Wave."

Meanwhile, another group of young people were tired of all the angst of the late 60's and early 70's protest movements, and they just wanted to have a good time and dance. They originally went into warehouses in New York and set up discotheques where they could play their rhythmic dance music that was fused with R&B, soul, and Latin sounds. However, that music spread to Miami and eventually Europe, and it became known as disco. It was mostly known as a fun but weird niche scene, but it blew up after the release of the movie Saturday Night Fever and the founding of Studio 54. Soon the whole world went crazy for disco, but after the Disco Demolition Night in 1979, the election of Ronald Reagan in 1980, and the tragic rise of AIDS shortly thereafter, disco was considered persona non grata and driven underground, where it would morph into something different.

But not all people could afford to go to the discos in the late 1970's, particularly those in the ravaged south Bronx, Harlem, and Queens neighborhoods. A young DJ named Kool Herc went into neighborhoods where he formed block parties that consisted of music that played extended cuts of records which had artists speaking over the tracks in style that would be called rap. Soon, a few more artists like the Cold Rush would catch on to this new music style, but blew up after the release of the Sugar Hill Gang's "Rappers Delight" and "The Message" by Grandmaster Flash and the Furious Five.

1983-1991: The Big 80's. Michael Jackson released his seminal album Thriller in late 1982 while Prince released 1999 also around the same time. But both of those albums would blow up in 1983, and Thriller became the best selling album of all time. Meanwhile, Prince became a superstar, and new wave acts like Sting and The Police, Duran Duran, and U2 would also become superstars. Likewise, a new generation of female artists would become pop superstar divas, and they were led by Madonna, Whitney Houston, Janet Jackson (Michael's sister), and later Mariah Carey.

But it wasn't just pop music that was rising to stratospheric levels. A new wave of metal acts were embraced by the mainstream, and the success of bands like Def Leppard, Quiet Riot, Motley Crue, and Bon Jovi led to the explosion of "hair metal," known as such due to the wild, larger than life hairstyles of those artists. What separated these artists from their metal predecessors was that their music was more pop oriented, and women comprised half of their fans.

1992-1996: Grunge, gangsta rap, and Alternative Nation. While the 80's music reveled in its bigness and decadence, it came to an abrupt halt towards the end of 1991, thanks to a new generation of artists that were about being real and rejecting the excesses of the past decade. It began in Seattle with the group Nirvana, but was also pushed by groups like Soundgarden, Alice In Chains, and Pearl Jam. Their sound was a grungy mix of heavy guitar and sonic feedback, and their lyrics were considerably downbeat, thus being known as "grunge". Other artists picked up on this, and while most of them didn't do the grungy sound, they kept the lyrical sentiment in place and formed their own alternative musical sytle to the mainstream.

Meanwhile, hip-hop music became decidedly more harsh, reflecting the realities of poverty and the ravages of the drug trade that these artists were surrounded by. It began in the late 1980's with Public Enemy and N.W.A. but blew up with Ice-T, 2Pac, Biggie Smalls, and the Wu-Tang Clan. Since many of these young hip-hop artists were previously in gangs or the drug trade, their music became known as "gangsta rap." However, the music would go down in flames, or more appropriately, a hail of bullets.

1997-2002: The TRL generation, part 1 - Millennials rising. After grunge went away due to the deaths of Kurt Cobain and the breakups of other bands; gangsta rap went away due to the murders of 2Pac and Biggie Smalls, and the economy improved due to the rise of the internet, a new generation of artists came onto the scene. They were led by the Spice Girls and Hanson, and their music was decidedly poppy and upbeat. But it really blew up with the rise of the Backstreet Boys, 'Nsync, 98 Degrees, Britney Spears, Destiny's Child, and Chrsitina Aguilera. Of course, these artists became mega-superstars after appearing on the wildly popular MTV TV program Total Request Live, or TRL as it became to be known. The show was so influential that older artists like Madonna, U2, Mariah Carey and the Foo Fighters, and even movie stars and sports heroes had to appear on the show to stay popular.

As for hip-hop, the starkness and bleakness of the early and mid-1990's that fuled gangsta rap was replaced by the decadence of bling. Part of that reflected the improving economy, but much of that was fueled by hip-hop becoming fully embraced by the mainstream. They were led by Jay-Z and P. Diddy, but Eminem became the great white hip-hop hope due to his authentic delivery and his otherwise introspective lyrics.

2003-2007: The TRL generation, part 2 - Crunk across America and American Idols. By 2003, both Justin Timberlake and Beyonce Knowles had left their respective groups to become solo superstars in their own right. Hip-hop reached its peak in popularity thanks to Eminem, Outkast, and a rising young hip-hop superstar from the midwest named Kanye West. It was also being affected by a southern-style hip hop groove known as crunk, and it now only invaded hip-hop music but pop music as well. Ironically, TRL, the program that launched many of these artists into superstardom, was waning in influence thanks to the internet and the rise of nascent social media sites like Aimster, Friendster, and Livejournal, which in turn would be eclisped by Myspace and new sites like YouTube, Facebook, and Twitter. TRL aired its final episode in October, 2007.

Also during this time, reality TV became huge on television, and while some people were excited to see people roughing it out on a desert island or jungle, eating worms and being put in a box full of snakes, or competing for the affections of a potential Mr. Right with 20 other women, the most popular of the reality TV shows was American Idol. While some critics scoffed at the idea that a pop superstar could be launched from a TV show, it became a phenomenon thanks to the combination of incredibly talented singers and nearly twice as many tone deaf wannabe stars. It did launch the careers of Kelly Clarkson, Clay Aiken, Jennifer Hudson, Carrie Underwood, Chris Daughtry, and Phillip Phillips.

2008-present: The social media generation. By 2008, social media sites like Facebook, Twitter, and YouTube were making their presence felt in all media, and music was no exception. Suddenly, popularity was no longer just about how many albums one sold or how many concerts one sold out, but how viral was your song or video. Sure, some acts became novelty one hit wonders (Rebecca Black - "Friday", Psy - "Gangham Style"), but other acts like Justin Bieber and Macklemore and Ryan Lewis became superstars due to their built-in viral followings on social media. Also, established artists like Lady Gaga, Katy Perry, Taylor Swift, and Miley Cyrus often made music and videos which were designed to take advantage of social media.