Modern is a constructed format that allows expansion sets and core sets from Eighth Edition forward, save for the Modern ban list. The modern format thus encompasses all cards that have been printed in a set using the modern Card Frame. The format was officially codified on August 12, 2011, when the format of Pro Tour Philadelphia was changed from Extended to Modern.
Constructed decks must contain a minimum of sixty cards. There is no maximum deck size; however, you must be able to shuffle your deck with no assistance. If a player wishes to use a sideboard, it must contain exactly fifteen cards.
With the exception of basic land cards, a player's combined deck and sideboard may not contain more than four of any individual card, counted by its English card title equivalent. All cards named Plains, Island, Swamp, Mountain, and Forest are basic, as are their snow-covered counterparts — Snow-Covered Plains, Snow-Covered Island, Snow-Covered Swamp, Snow-Covered Mountain, and Snow-Covered Forest. 
As of May 5th, 2013, cards from the following sets are permitted in Modern tournaments:
† This includes timeshifted cards in the old time frame
- See also: Timeline of DCI bans and restrictions.
As of May 3rd, 2013, the following cards are banned in Modern tournaments:
- Modern Death Cloud Rock deck
- Modern Delver deck
- Modern Infect Combo deck
- Modern Living End deck
- Modern Melira deck
- Modern Mystical Teachings deck
- Modern Pyromancer deck
- Modern Zoo deck