"I was ready for the next big challenge," Epstein said at a news conference Tuesday at Wrigley Field, where he was introduced as the president of baseball operations.
He leaves the Red Sox after nine seasons as the general manager, signing a five-year deal with the Cubs, believed to be worth $18.5 million.
"To me," he said, "baseball is better with tradition, baseball is better with history, baseball is better with fans who care, baseball is better with ballparks like this, baseball is better during the day, and baseball is best of all when you win. That, ultimately, is why I'm here today."
Epstein vowed to build "a foundation of sustained success," which he said starts with a commitment to player development. He was impressed by the Cubs' aggressive approach in the First-Year Player Draft, and said it made him take notice that the team's owners, the Ricketts family, also believed in developing from within.
Cubs chairman Tom Ricketts began his search in August after Jim Hendry was dismissed as general manager. Ricketts said he studied other teams and how they went about their business, as well as talked to about 20 people to find the right person to head the Cubs' baseball operations department.
Ultimately, he settled on Epstein, and Ricketts said he knew after 10-15 minutes into their first conversation that he had targeted the right person.
Giddy Cubs fans have been celebrating since word got out. On Tuesday, they could purchase an "Epstein" No. 12 Cubs jersey across the street from Wrigley Field.
"It's unique," Epstein said of the Cubs. "Boston's not Chicago, Chicago's not Boston. Every city, every market has its own personality, its own idiosyncracies. I don't pretend to understand them all yet.
"Having grown up in Boston, I have a better feel for that market. I'm a baseball fan. When I was with the Padres, I'd come through Chicago and we'd go through Wrigley Field and play the Cubs. I got a sense of what things were like here."
What Epstein inherits is a team that finished fifth in the National League Central in back-to-back seasons, and generations of fans who have been waiting to pop champagne since 1908, the last time the Cubs won a World Series. It's the longest drought in professional sports.
"I'm not saying the timetable is immediate," Epstein said. "Over time, we'll get this thing done. It's not going to happen overnight. But we'll take every opportunity to win seriously."
Epstein has talked to manager Mike Quade and wants to meet with him face to face to discuss his "vision." Epstein did not immediately change managers when he took over the Red Sox in November 2002, keeping Grady Little for one season before hiring Terry Francona for the '04 season.
When Francona decided to leave after this season, Epstein realized it may be time to move on. He had set a 10-year time frame in Boston, and didn't feel it was fair to the Red Sox if he interviewed the next manager, and then left.
"The biggest factor of the exact timing was all of a sudden we looked up and had to hire a new manager," Epstein said. "I remember how it was for me, as a young GM, to run the interview process and bond with Terry Francona during that time.
"I didn't think it was right for me as someone likely to make a transition in the next year to run the search process and hire a new manager," he said. "It would've been awkward at best, disastrous at worst."
When Epstein took over as the Red Sox GM in November 2002, he pledged to transform the team into a "scouting and player development machine," and also got the Red Sox into the postseason. That's what the Ricketts want for their team.
Epstein's not afraid to make changes. The Red Sox roster at the end of '03 had 17 players who were not on the team the previous year, and nine of those were not even in the organization when the regular season began.
Epstein's first deal with the Cubs will essentially be for himself. When the Cubs and Red Sox announced Friday night that Epstein had resigned as Boston's GM, compensation had not been determined.
He will pick up negotiations with Ben Cherington, who was introduced as Boston's GM on Tuesday at Fenway Park. MLB Commissioner Bud Selig has set Nov. 1 as a deadline for the two sides to settle the matter.
The Cubs did receive permission from the Padres to talk to Jed Hoyer about also relocating to Chicago and becoming the general manager. Epstein and Hoyer were together on the Red Sox, and led the team to two World Series championships in 2004 and '07.
The Cubs also may add Jason McLeod, who was the Red Sox's scouting director, and is currently the Padres' assistant GM. Those hires were expected to be done after the World Series ends.
Epstein started watching baseball on TV when he was 2 years old, and was a Whiffle ball whiz at Central Park in New York growing up. His mother tells the story of how he knew at the age of 12 that he would work in baseball.
"It's always been my passion -- love the game, love the competitiveness," he said.
Now, Epstein starts anew. And so do the Cubs.
Carrie Muskat is a reporter for MLB.com. Muskat writes a blog, Muskat Ramblings, and you can follow her on Twitter@CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.