Hybrid Theory is the first EP by Linkin Park, then known as Hybrid Theory. Recorded and self-released in May 1999, later that year, the band changed its name to Linkin Park and Hybrid Theory was re-mixed and remastered for another limited release in 2001 (with minor changes) in the credits and artwork for members of Linkin Park's fan club. Only about 1,000 copies were made and released. Original versions of the EP are hard to find and because of this, many bootleg or fake versions of it have been made.

The financing for the original pressing was obtained through a publishing deal with Zomba Music Publishing after one of its representatives, Jeff Blue, heard the band playing at the Whisky a Go Go. Initial sales were merely a handful but the band persisted. They began plugging the album in chatrooms and sending free copies to anyone who expressed interest. An informal 'street team' developed from the on-line community to spread the music. With Dave Farrell's temporary departure from the band, Kyle Christener took over the bass position. However, he did not stay for long and did not take part in the band's debut album.

There is an untitled hidden track placed at the end of the EP, sometimes referred to as "Ambient" or "Secret". Elements of this song were sampled for the track "Session" on Linkin Park's second album Meteora.

Some songs included on the EP appeared in later releases by Linkin Park. "Step Up" appeared on the "In the End, Pt. 2" single. Samples from that song are also featured in the remix for "Cure for the Itch", titled, "KYUR4 TH ICH" on Linkin Park's Reanimation remix album, and The X-Ecutioners' "It's Goin' Down". "And One" and "Part of Me" were included on the 2008 Linkin Park compilation EP Songs from the Underground, released exclusively through Best Buy. A re-recorded version of "High Voltage", with a different chorus and some new lyrics, appears on the "One Step Closer" single and as a bonus track as well as on some versions of Linkin Park's debut album. Evidence's remix, featuring Pharoahe Monch and DJ Babu, appears on Reanimation. During the 2004 edition of that year of the Projekt: Revolution, the band played live a medley of "Step Up", "Nobody's Listening" (from Meteora) and "It's Goin' Down", later registered on the LPUnderground 4.0. The track "Announcement Service Public" from LPUnderground 6.0 is a remix of the secret track known as "Ambient". Also, the name Hybrid Theory was later used as the title of Linkin Park's debut studio album. (wiki)