The Division Series was implemented in 1981 as a result of a midseason strike with first place teams before the strike taking on the first place teams after. After 1993, it was implemented for good when Major League Baseball restructured each league into three divisions, but their next playing was in 1995 due to the cancellation of the 1994 playoffs. Previously, because of a players' strike in 1981, a split-season format forced a divisional playoff series, in which the Montreal Expos won the Eastern Division series over the Philadelphia Phillies three games to two while the Los Angeles Dodgers beat the Houston Astros three games to two in the Western Division. The team with the best overall record in the major leagues, the Cincinnati Reds, failed to win their division in either half of that season and were controversially excluded, as were the St. Louis Cardinals, who finished with the NL's second-best record. The Atlanta Braves have currently played in the most NL division series with eleven appearances. The Pittsburgh Pirates (whose finished with a losing record from 1993-2012) were the last team to make their first appearance in the NL division series, making their debut in 2013 after winning the 2013 National League Wild Card Game.
From 1998 to 2011, the wild-card team was assigned to play the division winner with the best winning percentage (outside of their own division) in one series, and the other two division winners met in the other series. However, if the wild-card team and the division winner with the best record were from the same division, the wild-card team played the division winner with the second-best record, and the remaining two division leaders played each other. Beginning with the 2012 season, the wild card team that advances to the Division Series was to face the number 1 seed, regardless of whether or not they are in the same division. The two series winners move on to the best-of-seven NLCS. The winner of the wild card has won the first round 7 out of the 11 years since the re-alignment and creation of the NLDS. According to Nate Silver, the advent of this playoff series, and especially of the wild card, has caused teams to focus more on "getting to the playoffs" rather than "winning the pennant" as the primary goal of the regular season.
Initially, the best-of-5 series played in a 2-3 format, with the first two games set at home for the lower seed team and the last three for the higher seed. Since 1998, the series has followed a 2-2-1 format,  where the higher seed team plays at home in Games 1 and 2, the lower seed plays at home in Game 3 and Game 4 (if necessary), and if a Game 5 is needed, the teams return to the higher seed's field. When MLB added a second wild card team in 2012, the Division Series re-adopted the 2-3 format due to scheduling conflicts. It will revert to the 2-2-1 format from 2013 onwards.