The 2003 National League Championship Series (NLCS) was a Major League Baseball playoff series played from October 7 to October 15 to determine the champion of the National League, between the Central Division champion Chicago Cubs and the wild-card qualifying Florida Marlins. The Cubs, by virtue of being a division winner, had the home field advantage. The Marlins came back from a three games to one deficit and won the series in seven games, advancing to the World Series against the New York Yankees.

The two teams were victorious in the NL Division Series (NLDS), with the Cubs defeating the Eastern Division champion Atlanta Braves three games to two, and the Marlins defeating the Western Division champion San Francisco Giants three games to one.

The series is most remembered for events that unfolded in the top of the eighth inning of Game 6. Coming into the game, the Cubs had taken two out of the three games in Miami, with the final two games at Wrigley Field in Chicago. The Cubs also had their best two pitchers, Mark Prior and Kerry Wood, slated to start the final two games. With the Cubs leading 3–0 and just five outs away from their first World Series since 1945, Steve Bartman, a fan, reached for the foul ball hit by Luis Castillo off Prior, preventing Cubs outfielder Moisés Alou from catching it. Castillo proceeded to walk and Prior and the Cubs never recovered from the incident. Aided by Castillo's walk and later an error by Cubs shortstop Alex S. Gonzalez on a potential double-play grounder, the Marlins went on to score eight runs in the inning and won the game 8–3. The Marlins went on to win Game 7 and then to defeat the New York Yankees in the World Series.

In Major League Baseball, the National League Championship Series (NLCS) is a round in the postseason that determines who wins the National League pennant and advances to Major League Baseball's championship, the World Series, facing the winner of the American League Championship Series. The reigning National League Champions are the St. Louis Cardinals.

Prior to 1969, the National League champion (the "pennant winner") was determined by the best win-loss record at the end of the regular season. There were four ad hoc three-game playoff series due to ties under this formulation (in 1946, 1951, 1959 and 1962). (The American League had to resolve a tie in 1948, but used a single-game playoff for that.)

A structured postseason series began in 1969, when both the National and American Leagues were reorganized into two divisions each, East and West. The two division winners within each league played each other in a best-of-five series to determine who would advance to the World Series. In 1985, the format changed to best-of-seven.

The League Championship Series (LCS) is the official name for a round of postseason play in Major League Baseball which has been conducted since 1969. In 1981, and since 1995, the two annual series have matched up the winners of the Division Series, and the winners advance to meet in the World Series.

The League Championship Series was promulgated in 1969, when both the National League and the American League increased in size from ten teams to twelve with the addition, via expansion, of the Montreal Expos and San Diego Padres to the former and the Kansas City Royals and Seattle Pilots (now the Milwaukee Brewers of the NL) to the latter. Concomitant with this, both leagues formed Eastern and Western Divisions, the first-place teams from which faced off in the LCS.

Originally, the League Championship Series were best-of-five, but were lengthened to best-of-seven in 1985. Since 1995 the LCS has matched up the winners of the Division Series, which were added when both leagues realigned into three divisions, the winners thereof plus one wild card team in each league participating in the latter series, which is a best-of-five (this change was actually supposed to take place in 1994, but postseason play was not held in that year due to a players' strike).

Thomas Wade "Thom" Brennaman (born September 12, 1963) is an American sportscaster, and the son of current Cincinnati Reds radio sportscaster Marty Brennaman.

After graduating in 1982 from Cincinnati's Anderson High School Thom attended Ohio University, where he was president of the Beta Kappa Chapter of Beta Theta Pi fraternity. He entered college uncertain of whether to follow in his father's footsteps and become a broadcaster. While at Ohio U he joined station WATH, developing his own love for radio. After graduating in 1986, Brennaman worked as a sports reporter/anchor for WLWT-TV, the NBC affiliate in Cincinnati. During this same period, he worked as the television play-by-play announcer for the Cincinnati Reds along side Major League Baseball Hall of Famer Johnny Bench. In the early 1990s he did Chicago Cubs broadcasts, sometimes with Hall of Fame broadcaster, Harry Carry. In 1994 he was hired by Fox Sports to call the network's National Football League and Major League Baseball telecasts. He served as the first television voice for the Arizona Diamondbacks, and left after the 2006 season to join his father Marty in Cincinnati.