A thrash metal band formed by Dave Mustaine in 1983 after he left (got kicked out of) Metallica. Megadeth has gone through many line up changes with the only two stable members being Dave Mustaine (guitar/vocals) and Dave Ellefson (bass). The original line-up included Chris Poland on guitar and Gars Samuelson on drums. Before recording their third album So Far, So Good...So What! Mustaine fired Poland and Samuelson and hired guitarist Jeff Young and drummer Chuck Behler. In 1990 Mustaine fired Young and Behler and replaced them with guitarist Marty Friedman and drummer Nick Menza. This is the line-up most associate with Megadeth. The 1999 release Risk featured a new drummer, Jimmy DeGrasso who used to play with Suicidal Tendencies. I've also heard Marty Friedman has left to pursue a solo career.

Most of reason this band was so unstable can be attributed to Dave Mustaine and his drug and alcohol problems. These were the reasons he was kicked out of Metallica and for many years it only got worse after that. There were always many stories and rumors about his problems circulating about the metal scene. Then in 1990 he was arrested for driving under the influence and entered a rehabilitation program. He has apparently been clean and sober since then. IIRC he had a wife and kid at the time and once he got sober and saw how his problems had effected them they were his motivation for staying that way.

In my high school days I was a thrash/speed metal fan. I don't think I bought another Megadeth album after Countdown to Extinction. Although I don't think Youthanasia was a bad album I was growing out of my metal phase of life, still I may own it I would have to dig up my old metal albums. Hmmm, making this writeup has given me a hankering to hear Hanger 18 and the rest of Rust In Peace. Which in my humble opinion is Megadeth's best album. I guess I'll be digging through CDs soon.

Albums:
Super Collider -- Universal Music Group (2013)
Thirteen -- Roadrunner Records (2011)
Rust In Peace Live -- Shout! Factory (2010) (live album)
The Big 4 Live from Sofia, Bulgaria -- Universal Music Group (2010) (live album)
Endgame -- Roadrunner Records (2009)
That One Night: Live in Buenos Aires -- Image Entertainment (2007) (live album)
United Abominations -- Roadrunner Records (2007)
The System Has Failed -- Sanctuary Records (2004)
Rude Awakening -- Sanctuary Records (2002) (live album)
The World Needs A Hero -- Sanctuary Records (2001)
Risk -- Capitol Records (1999)
Cryptic Writings -- Capitol Records (1997)
Youthanasia -- Capital Records (1994)
Countdown to Extinction -- Capitol Records (1992)
Rust in Peace -- Capitol Records (1990)
So Far, So Good...So What! -- Capitol Records (1988)
Peace Sells...But Who's Buying? -- Capitol Records (1986)
Killing Is My Business... And Business Is Good! -- Combat (1985)

Big ups to TW for sending some additional info my way (pointed out that the band had a new album out and more)