The Curse of "Billy The Goat" Sianis
As the story goes, Billy Sianis, a Greek immigrant (from Paleopyrgos, Greece), who owned a nearby tavern (the now-famous Billy Goat Tavern), had two $7.20 box seat tickets to Game 4 of the 1945 World Series between the Chicago Cubs and the Detroit Tigers, and decided to bring along his pet goat, Murphy (or Sinovia according to some references), which Sianis had restored to health when the goat had fallen off a truck and subsequently limped into his tavern. The goat wore a blanket with a sign pinned to it which read "We got Detroit's goat". Sianis and the goat were allowed into Wrigley Field and even paraded about on the playing field before the game before ushers intervened and led them off the field. After a heated argument, both Sianis and the goat were permitted to stay in the stadium occupying the box seat for which he had tickets. At this point, Andy Frain (head of Wrigley Field's hired security company at the time), waved the goat's box-seat ticket in the air and proclaimed "If he eats the ticket that would solve everything.". However, the goat did not. Before the game was over, Sianis and the goat were ejected from the stadium at the command of Cubs owner Philip Knight Wrigley due to the animal's objectionable odor. Sianis was outraged at the ejection and allegedly placed a curse upon the Cubs that they would never win another pennant or play in a World Series at Wrigley Field again because the Cubs organization had insulted his goat, and subsequently left the U.S. to vacation in his home in Greece.
The Cubs lost Game 4 and eventually the 1945 World Series, prompting Sianis to write to Wrigley from Greece, saying, "Who stinks now?" Following a third-place finish in the National League in 1946, the Cubs would finish in the league's second division for the next 20 consecutive years. This streak finally ended in 1967, the year after Leo Durocher became the club's manager. Since that time, the supposedly cursed Cubs have not won a National League pennant or played in a World Series " the longest pennant drought in Major League history. Sianis died in 1970.
 Attempts to break the curse
Billy Sianis's nephew Sam Sianis has been brought out on the field with a goat multiple times in attempts to break the curse: on Opening Day in 1984 and 1989 (the Cubs won the division both years), in 1994 to stop a home losing streak, and in 1998 for the wild card play-in game (which the Cubs won). 
A group of Cubs fans headed to Houston in 2003 with a Billy Goat named "Virgil Homer" and attempted to gain entrance to Minute Maid Park. After they were denied entrance, they unfurled a scroll and read a verse proclaiming they were "reversing the curse".  Houston faded down the stretch allowing the Cubs to win the division. The Astros did win the NL pennant in 2005. The Cubs came within a game of the World Series in 2003, but apparent fan interference from Steve Bartman is believed to have led to the Cubs losing the next two games, and the pennant, to the Florida Marlins.
Before the 2004 season the Steve Bartman ball was ceremonially destroyed at Harry Caray's Restaurant in Chicago.
 The cure
According to an interview with Sam Sianis, William Sianis' nephew, the Curse of the Billy Goat can only be dispelled by the Chicago Cubs organization showing a true sincere fondness for goats, sincerely allowing them into Wrigley Field because they like them, and not simply for publicity reasons.
 Former Cubs who won a World Series title elsewhere
Another factor that may play a role in the curse is the number of players who won World Series titles after leaving the Cubs. These players include Andy Pafko (who, coincidentally, played in the 1945 World Series as a Cub), Smoky Burgess, Don Hoak, Dale Long, Lou Brock, Lou Johnson, Jim Brewer, Moe Drabowsky, Don Cardwell, Ken Holtzman, Billy North, Bill Madlock, Manny Trillo, Rick Monday, Burt Hooton, Bruce Sutter, Willie Hernández, Joe Niekro, Dennis Eckersley, Joe Carter, Greg Maddux, Joe Girardi, Glenallen Hill (after his second stint with the Cubs), Luis González, Mike Morgan, Mark Grace, Mark Bellhorn and Bill Mueller. Dontrelle Willis and Jon Garland were traded as minor leaguers.
 Former Cubs cursing other teams
Conversely, the "Ex-Cubs Factor" seemed to plague many a post-season qualifier that had too many former Cubs. This theory reached its zenith in 1990, when the factor "predicted" that the Oakland Athletics were "doomed" in that year's World Series, and the A's were swept by the Cincinnati Reds in a stunning upset. In the 2001 World Series, however, the Arizona Diamondbacks faced the Yankees with three ex-Cubs on their roster, and not only won the Series in dramatic fashion, but won it on a rally started by Mark Grace, an ex-Cub, effectively discrediting the "Ex-Cubs" theory.
Boston Red Sox first baseman Bill Buckner, who was blamed for Boston's 1986 World Series loss after a routine ground ball rolled through his legs, was also a former Cub. It has been recently uncovered that at the time of the play (and in many other instances), Buckner was wearing an old and tattered Chicago Cubs batting glove under his fielding glove.
Former Cub pitcher Mike Krukow (who went on to play for the San Francisco Giants and is currently a broadcaster for them) is alleged to be the source of the legendary "Krukow Kurse". The "Krukow Kurse" is used to explain the Giant's fifty-plus year failure to win the World Series while in San Francisco. Before the start of each season, Krukow states his usual optimistic prediction- during his radio show-that the Giants have a chance to ultimately win the World Series. Once Krukow stops making such preseason predictions- says the legend- the Giants will, in fact, win the World Series.
Another former Cub, Mitch Williams, also suffered World Series heartbreak in 1993, when he gave up a legendary walk-off home run to Joe Carter of the Toronto Blue Jays in the ninth inning of Game 6, handing the World Championship to Toronto. Coincidentally, Carter was also a former Cub, but he and Williams had not been teammates in Chicago.