Megadeth was formed in late 1983 when Dave Mustaine (formerly of Metallica) threw a potted plant through the window of David Ellefson's apartment because Ellefson was practicing his bass too loud. When the band was formed Dijion Carruthers was their drummer and Greg Handevidt on guitars. They quickly replaced Carruthers with drummer Lee Rausch, recorded a demo and secured a few gigs in the San Francisco Bay area in February, 1984. Handevidt was fired for incompetence (he later formed Kublai Khan) They quickly found second guitarist Kerry King (Slayer (US)), who played Megadeth's first three gigs: February 15th, 18th, and 19th, of 1984.
The band name was selected from a term referring to a million casualties, as a result of Nuclear Fallout. Some senator used the term. The extra "a" was dropped by the band. The name Fallen Angel was considered, as it was the name of a short-lived Dave Mustaine/Matt Kisselstein (bass) project in 1983.
King left later that month when Mustaine wanted him to choose between Megadeth and Slayer (US). Then, Chris Poland was brought in, and more gigs were played, starting on April 15th, 1984. A demo was recorded with this lineup, and then Lee Rausch left (he would later that year show up in Dark Angel (US)), and was replaced by Gar Samuelson. This lineup recorded the first two albums.
There were troubles with Chris Poland. He was replaced with Mike Albert for part of the 1985 tour, and was finally fired in 1987, for stealing Mustaine's guitars and other transgressions (read the lyrics to "Liar" for a somewhat biased but not entirely untrue account), and Gar Samuelson left as well (his parting wasn't entirely amicable either). Chuck Behler came in immediately, and several guitarists were tried out, including Jay Reynolds of Malice (US), who played a few gigs on the 1987 tour, before giving way to Jeff Young. This lineup recorded So Far, So Good... So What, but after a subsequent tour, both Young and Behler were fired.
Nick Menza, Chuck's drum tech, went behind the kit himself in 1989, and the band recorded, as a trio, "No More Mister Nice Guy", an Alice Cooper cover for the Shocker soundtrack. After tryouts that included Darrell Roberts (W.A.S.P.) and a 16 year old Jeff Loomis (Nevermore (US)), Marty Friedman of Cacophony was hired in February of 1990, and this lineup lasted for about 8 years and produced four albums and most of the songs on an EP, before Nick Menza left due to a knee injury.
Jimmy DeGrasso (of Y&T and White Lion) was brought in, and Risk was recorded. Then, Marty Friedman left, citing a desire to play Japanese pop music, while Mustaine wanted to return to heavy metal. Thus, Al Pitrelli of Savatage, as well as Trans-Siberian Orchestra and C.P.R. joined the band. A new album, The World Needs A Hero, was recorded and a tour with Iced Earth launched afterwards.
Dave Mustaine, citing an arm injury that left him unable to play guitar effectively, left Megadeth in April 2002, and the band was dissolved. It is rumoured that Ellefson, Menza and Friedman are cooperating on a project together, and of course rumours of a Megadeth reformation are constant, as Dave Mustaine is just a little unpredictable.
And, as it turns out, he was. On January 13, 2004, Dave Mustaine, following his alleged "religious awakening," announced his plans to re-form Megadeth with former guitarist Chris Poland. Unfortunately, Dave Ellefson, Al Pitrelli and Jimmy DeGrasso did not want to rejoin and went back to working on their other projects, and forming their own band, which is now auditioning vocalists. Dave Ellefson has also joined Soulfly.