From the Big Ten website:
http://www.bigten.org/trads/big10-trads.html
The history of the Big Ten traces back 120 years to the Palmer House hotel in Chicago, where on January 11, 1895, then-Purdue president James H. Smart and leaders from the University of Chicago, University of Illinois, University of Michigan, University of Minnesota, Northwestern University and University of Wisconsin set out to organize and develop principles for the regulation of intercollegiate athletics.
On February 8, 1896, one faculty member from each of those seven universities met at the same Palmer House and officially established the mechanics of the conference, which was officially incorporated as the “Intercollegiate Conference Athletic Association” in 1905.
Expansion:
Indiana University and the State University of Iowa became the eighth and ninth members in 1899. In 1908, Michigan briefly withdrew its membership, and in 1912 Ohio State University joined the conference, bringing its membership total back to nine. Upon Michigan’s return in 1917, the conference was first referred to as the “Big Ten” by media members, and that name was eventually incorporated in 1987.
After nearly 30 years with 10 members, the conference consolidated to nine schools when the University of Chicago formally withdrew its membership in 1946. Michigan State College (now Michigan State University) was added to the Big Ten three years later, bringing the number of affiliated conference schools to 10 once again.
In December of 1989, the conference agreed in principle to invite Pennsylvania State University for membership. On June 4, 1990, the Council of Presidents officially voted to integrate Penn State into the conference, giving the Big Ten 11 members.
On June 11, 2010, the Big Ten Council of Presidents/Chancellors (COP/C) approved a formal membership application by the University of Nebraska, expanding the conference to 12 institutions. Nebraska officially joined the Big Ten on July 1, 2011.
The conference expanded its footprint further in 2012 when the COP/C approved formal membership applications from the University of Maryland and Rutgers University on November 19 and 20, respectively. Maryland and Rutgers became official Big Ten members on July 1, 2014, giving the conference almost 9,500 students participating in intercollegiate athletics and more than 11,000 participation opportunities on 350 teams.
On June 3, 2013, the Big Ten announced that Johns Hopkins University had been accepted as the conference’s first sport affiliate member. Johns Hopkins’ addition in men’s lacrosse gave the Big Ten six institutions sponsoring the sport, allowing the debut of men’s lacrosse as an official conference sport and building on the Big Ten’s tradition of broad-based sports competition. On June 17, 2015, the conference announced that Johns Hopkins was accepted as a sport affiliate member for women's lacrosse beginning with the 2016-17 academic year.
More than 120 years after its inception, the Big Ten remains a national leader in intercollegiate athletics on and off the field. Big Ten programs have combined to win more than 450 team and 1,800 individual national championships, consistently taking home individual honors for athletic and academic accomplishments and fulfilling the Big Ten’s mission of academic achievement and athletic success.