From www.wizards.com
Updated February 2nd, 2014
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. (The five Ice Age block snow-covered lands -- {Snow-Covered Plains}, {Snow-Covered Island}, {Snow-Covered Swamp}, {Snow-Covered Mountain}, and {Snow-Covered Forest} -- are also basic lands. Note that snow-covered lands are permitted only in formats that allow expansions from the Ice Age block to be used.)
The following card sets are permitted in Modern tournaments:
Eighth Edition
Mirrodin
Darksteel
Fifth Dawn
Champions of Kamigawa
Betrayers of Kamigawa
Saviors of Kamigawa
Ninth Edition
Ravnica: City of Guilds
Guildpact
Dissension
Coldsnap
Time Spiral
Planar Chaos
Future Sight
Tenth Edition
Lorwyn
Morningtide
Shadowmoor
Eventide
Shards of Alara
Conflux
Alara Reborn
Magic 2010
Zendikar
Worldwake
Rise of the Eldrazi
Magic 2011
Scars of Mirrodin
Mirrodin Besieged
New Phyrexia
Magic 2012
Innistrad
Dark Ascension
Avacyn Restored
Magic 2013
Return to Ravnica
Gatecrash
Dragon's Maze
Magic 2014
Theros
Born of the Gods
Journey into Nyx
Khans of Tarkir
Fate Reforged
The following cards are banned in Modern tournaments:
2016 bans:
Splinter twin
Summer bloom
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Banlist
{Ancestral Vision}
{Ancient Den}
{Birthing Pod}
{Blazing Shoal}
{Bloodbraid Elf}
{Chrome Mox}
{Cloudpost}
{Dark Depths}
{Deathrite Shaman}
{Dig Through Time}
{Dread Return}
{Glimpse of Nature}
{Great Furnace}
{Green Sun's Zenith}
{Hypergenesis}
{Jace, the Mind Sculptor}
{Mental Misstep}
{Ponder}
{Preordain}
{Punishing Fire}
{Rite of Flame}
{Seat of the Synod}
{Second Sunrise}
{Seething Song}
{Sensei's Divining Top}
{Stoneforge Mystic}
{Skullclamp}
{Sword of the Meek}
{Treasure Cruise}
{Tree of Tales}
{Umezawa's Jitte}
{Vault of Whispers}
2015 Unbannings
{Golgari Grave-Troll}
Official information about Constructed formats and the Modern format can be found in the Magic: The Gathering Tournament Rules.
http://www.wizards.com/wpn/Events/Rules.aspx?category=magic:thegathering