"Fireflies" is the debut single from American electronica project Owl City's album Ocean Eyes. Frontman Adam Young wrote and produced the track, with Matt Thiessen also producing and providing guest vocals. Young described the song as "a little song about bugs and not being able to fall asleep at night." The song is built around a "bleepy" synthline and includes lyrics about insomnia, fireflies and summer.
The song topped the Billboard Hot 100 for two non-consecutive weeks. Internationally, the song also topped the charts in Australia, Belgium, Denmark, Finland, the Netherlands, Norway, Ireland, Sweden and the United Kingdom. "Fireflies" was Owl City's only top forty hit in the United States until three years later when "Good Time", a duet with Canadian singer Carly Rae Jepsen, charted at No. 8. It has been covered by Christina Grimmie, Cheryl Cole and others.
The song debuted on the Billboard Hot 100 in early-September at No. 97, and it reached No. 1 in its tenth week, becoming Owl City's first No. 1 single. The song stayed at No. 1 for two non-consecutive weeks, in the top ten for fifteen weeks and on the Hot 100 for 31 weeks. "Fireflies" contributed to sales of the album Ocean Eyes, and was cred as being responsible for its entry to the top ten on the U.S. Billboard 200. On the Billboard Hot 100 2009 year-end chart, it was ranked sixtieth. On the Billboard Hot 100 2010 year-end chart, it was ranked thirtieth. The RIAA certified "Fireflies" 3× Platinum in June 2010. By July 2018, the song had shifted 5,269,000 downloads in the United States, enough for a 4× Platinum certification.
The song attained success worldwide. In the United Kingdom, the song entered at No. 50 due to early download sales from a fake version that was leaked onto iTunes. The song would go on to make a 48-place jump to number two the following week, beaten only to the top by "Replay" by Iyaz. The following week, it rose to number one and topped the chart for three consecutive weeks. On January 2, 2011, it was revealed that "Fireflies" was the 20th most downloaded song of all time in the UK. As of September 2017, the song has sold over 844,000 copies in the UK.
In Australia, the song entered at No. 38 and on the week of January 10, 2010, it topped the chart. In Japan, the song peaked at No. 3 and was ranked sixteenth on the 2010 year-end chart, the highest ranking for an international song for 2010. It reached No. 1 in Denmark, Ireland, Sweden, Australia, the United Kingdom and the Netherlands (for 10 weeks) and the top ten in Austria, Belgium, Canada, Poland, Finland, Germany, New Zealand, Norway, Portugal, and Switzerland. The song was ranked 89th on VH1's 'Top 100 songs of the new millennium'.
The music video for "Fireflies" was directed by Steve Hoover. It features Adam Young playing the song on a Lowrey spinet organ in a toy-filled bedroom, where most of the toys (including an astronaut, a Tyrannosaurus rex, a Speak & Spell, toy cars including one based on Brum, and a blimp) come to life. Most of the toys are older model toys, with most of them from the 1970s and 1980s (the exceptions being a Robosapien and a Roboraptor). There are also vintage household devices such as a black and white television and a record player. As Young plays the organ, he activates a "magic" button on his organ and the aforementioned toys come to life. As the song progresses towards the end toys return to normal one by one until he turns off the magic button. The video ends with him turning off his organ as the camera cuts to black.
The video had an exclusive premiere on Myspace, but was leaked onto Dailymotion sometime before and YouTube soon after. As of April 1, 2020, the video has received more than 350 million views.
Dancehall artist Busy Signal recorded a version of the song entitled "Between Eyes".
In May 2017, the song was revitalized as an Internet meme, usually involving the song being played at an extremely loud volume or being remixed to fit a certain theme. Another variant of the meme involved writing a pun for the line "You would not believe your eyes, if ten million fireflies". The song received further notability in June when Owl City was asked to interpret the lyric "I get a thousand hugs from 10,000 lightning bugs."