Mega Millions is a US Multi-jurisdictional $1 Lottery Game
Since it replaced The Big Game in May 2002, Mega Millions' advertised jackpots have started at $12 million, paid in 26 yearly installments (unless the cash option is chosen), increasing when there is no jackpot winner.
Reflecting common practice among American lotteries, the jackpot is advertised as a nominal value of annual installments. A cash value option, when chosen by a jackpot winner, pays the approximate present value of the installments.
Mega Millions currently uses a 1/56 (white balls) + 1/46 (the Mega Ball) double matrix to select its winning numbers.
Each game costs $1; in 43 of the 44 Mega Millions jurisdictions (California is the exception), there is an option, called Megaplier (such games are $2 each) where non-jackpot prizes are multiplied by 2, 3, or 4.
The Megaplier was made available to all Mega Millions jurisdictions in January 2011; it began as a Texas-only option.
Mega Millions is drawn at 10:59 p.m. Eastern time Tuesdays and Fridays, including holidays. Mega Millions is administered by a consortium of its original lotteries and the drawings usually are held in Atlanta, Georgia. supervised by the Georgia Lottery.
The largest advertised estimated jackpot in Mega Millions, and in American and world lottery history, is $656 million annuitized ($474 million in immediate payout) for the March 30, 2012 drawing.

The 2010 Expansion of Mega Millions and Powerball
On October 13, 2009, the Mega Millions consortium and Multi-State Lottery Association (MUSL) reached an agreement in principle to cross-sell Mega Millions and Powerball in US lottery jurisdictions with this combination of the 2 lotteries being referred to as the Mega Power Lottery by many users.
The expansion occurred on January 31, 2010, as 23 Powerball members began selling Mega Millions tickets for their first drawing on February 2, 2010; likewise, 10 Mega Millions members began selling Powerball tickets for their first drawing on February 3, 2010.
Montana (joining Mega Millions on March 1, 2010) was the first jurisdiction to add either game after the cross-sell expansion. Nebraska (March 20, 2010), Oregon (March 28, 2010), Arizona (April 18, 2010), Maine (May 9, 2010), Colorado, and South Dakota (the latter two on May 16, 2010) also have joined Mega Millions since the expansion.
With Louisiana joining Mega Millions in November 2011, Mega Millions and Powerball each are played in 44 jurisdictions, with 43 lotteries selling tickets for both games.
California is the only Mega Millions member that does not offer Powerball, while Florida is the only Powerball member that does not offer Mega Millions.
It is likely this cross-selling arrangement is a temporary measure, as lotteries investigate the possibility of merging the two games.