The NCAA Men's Division I Basketball Championship is a single elimination tournament held each spring featuring 65 college basketball teams in the United States. Colloquially known as March Madness (as the tournament takes place mainly during the month of March) or the Big Dance (as opposed to the now smaller and less prestigious NIT), the tournament takes place over 3 weeks at sites across the United States, and the national semifinals (the Final Four) have become one of the nation's most prominent sporting events. 

Since its 1939 inception (a brainchild of Phog Allen at the University of Kansas), it has built a legacy that includes dynasty teams and dramatic underdog stories. In recent years, friendly wagering on the event has become something of a national pastime, spawning countless "office pools" that attract expert fans and novices alike. All games of the tournament are broadcast on the CBS broadcast television network in the United States, except for the Opening Round game (or "play-in game" as it has been called), which aired on TNN in 2001, and ESPN since 2002. 

The tournament bracket is made up of conference tournament champions from each Division I conference, which receive automatic bids. The remaining slots are at-large berths, with teams chosen by an NCAA selection committee. The selection process and tournament seedings are based on several factors, including team rankings, win-loss records and RPI data. 

Two low-seeded teams (typically teams with poor records that qualified by winning their conference tournament championships) play the "opening round" game to determine which will advance into the first round of the tournament, with the winner advancing to play the top seed in one of the four regions. The opening Round game was added in 2001 and has been played in University of Dayton Arena in Dayton, Ohio each subsequent year. The opening round is considered part of the tournament and is often referred to as a "play-in" game. 

A Most Outstanding Player honor is awarded by the Associated Press at the end of each tournament. 

At 11 national titles, UCLA currently holds the record for the most NCAA Men's Division I Basketball Championships. University of Kentucky is second, with 7 national titles. 

A total of 65 teams qualify for the tournament played in March and April. Thirty-one of the teams earn automatic bids by winning their respective conference tournaments. Because the Ivy League does not conduct a post-season tournament, its regular-season conference champion receives an automatic bid. The remaining teams are granted at-large bids, which are extended by the NCAA Selection Committee. 

The tournament is split into four regions and each region has teams seeded 1–16, with the committee making every region as comparable to the others as possible. The best team in each region plays the #16 team, the #2 team plays the #15, and so on. The effect of this seeding structure ensures that the better a team is seeded, the worse-seeded their opponents will be. 

Two teams play a play-in game on the Tuesday preceding the first weekend of the tournament, with the winner of that game advancing to the main draw of the tournament and plays a top seed in one of the regionals. This game has been played at the University of Dayton Arena in Dayton, Ohio since its inception in 2001. This game originated in 2001 with the addition of the Mountain West Conference (and its automatic bid into the tournament) after the NCAA tournament committee opted to include an additional team and play in game in lieu of taking away one of the 34 "at-large" bids. These two teams share equally in the share of funds as if they had qualified for a first round game, and wins in the opening round game are considered wins in the NCAA tournament. Thus, properly, the tournament has 65 teams, although in practice most brackets only include the 63 teams, with one spot blank (to be filled in after the play-in game). Since no #16 seed has ever beaten a #1 seed in the men's championship, the result of the opening round game is largely deemed irrelevant for bracket-filling purposes. 

March Madness is a popular term for season-ending basketball tournaments played in March (Brent Musburger is generally regarded as the individual who first used that phrase in conjunction with the college tournament, using it during CBS Sports' coverage of the tourney back in 1982 - see below), especially those conducted by the National Collegiate Athletic Association (NCAA) and various state high school associations. The phrase was not associated with the college tournament in 1939, when an Illinois official wrote "A little March Madness [may] contribute to sanity." March Madness is also a registered trademark, held jointly by the NCAA and the Illinois High School Association. The trademark has sparked a pair of high-profile courtroom battles in recent years. 

March Madness refers to the frenzy these tournaments ignite among sports fans and, at least at the college level, sports gamblers. As it applies to college basketball, the term originally referred to the conference basketball tournaments, which occur in March just before the NCAA tournament begins, but in recent years has been used to refer to the NCAA tournament itself (the first weekend of which involves some 49 games, and which actually runs into early April). The term is now used in reference to both the men's and women's tournaments. The Big Dance also refers exclusively to the NCAA Tournaments to distinguish them from the conference tournaments and the NIT. 

As a tournament ritual, the winning team cuts down the net at the end of the regional championship game (and the national championship game). Each player cuts a single strand off of the net for themselves, commemorating their victory, with the head coach cutting the last strand and claiming the net itself. Furthermore, the regional champs (starting in 2007) receive a bronze plated NCAA Regional Championship trophy (previously given to only the Final Four teams that did not make the championship game), and the National Champions also receive a gold plated NCAA National Championship trophy along with a more elaborate marble/crystal trophy sponsored by Siemens. The loser of the championship game receives a silver plated National Runner-Up trophy for second place. 

The term Final Four refers to the last four teams remaining in the playoff tournament. These are the champions of the tournament's four regional brackets, and the only teams remaining on the tournament's final weekend. (The term has been applied retroactively to include the last four teams in tournaments from earlier years, when only two brackets existed.) 

Some claim that the phrase Final Four was first used to describe the final games of Indiana's annual high school basketball tournament. But the NCAA, which has a trademark on the term, says Final Four was originated by a Cleveland Plain Dealer sportswriter, Ed Chay, in a 1975 article that appeared in the Official Collegiate Basketball Guide. The article stated that Marquette University “was one of the final four” in the 1974 tournament. The NCAA started capitalizing the term in 1978, and turning it into a trademark several years later. 

The women's tournament starts with 64 teams, with no play-in game. The tournament proceeds by means of single elimination play on consecutive weekends in March at pre-selected sites in the United States. 

In the men's tournament, all sites are nominally neutral: teams are prohibited from playing tournament games on their home courts (though in some cases, a team may be fortunate enough to play in or near its home state or city). Under current NCAA rules, any court on which a team hosts more than three regular-season games is considered a "home court" (conference tournament games are not counted for this purpose). In the 2006 tournament, Villanova was able to play its first two games at the Wachovia Center in nearby Philadelphia, a venue where it had played three regular-season home games. A fourth home game at that facility would have disqualified them from playing there. However, some semi-"home" courts (such as George Mason playing its regional at the Verizon Center in Washington, D.C., not far from its campus in Fairfax, Virginia, in 2006) are mere quirks of scheduling and have been part of the tournament for years.