The reason that more and more institutions of higher education are accepting students in the spring is that it helps schools fill all their openings with quality candidates. Schools may waitlist “second-tier” applicants, granting them spring rather than fall admission, or they may allow new students and transfer students to directly apply for spring enrollment. Schools report little difference in performance of spring applicants, who, thanks to summer school and heavy course loads, frequently finish at the same time as fall entrants do.
To be sure whether a school offers spring admission, you should not only check its website but also the admission particulars for the exact program to which you intend to apply. Often, a specific college, school, department or major within a university will accept students in the spring, while another will not. Similarly, some degree tracks may only be open in the fall. For example, at Boston University, seven colleges are open to freshman. All accept new and transfer students in January. That said, the College of Fine Arts accepts spring applicants to its School of Music and School of Visual Arts, but not to its School of Theater. Also, students planning to pursue an Accelerated Liberal Arts degree, a Medical Education Combined Degree Program or a Dental Education Combined Degree Program at Boston University may only begin in the fall.
The first school that many people think of when you say “spring admission” is the University of Southern California. USC doesn't use a waitlist to determine who gets in in the spring, as do many other schools, but actually saves extra slots for applicants who want to attend in the spring. (See Reference 2.) At Colorado State University, students interested in spring admission need to have 24 credits to their name, which effectively disqualifies anyone but a transfer student. The University of Virginia allows spring admission, but not for all of its campuses and major choices. Again, check with your specific program or department, but spring admission is generally offered at the following schools: American University, Brandeis University, Colby College, Middlebury College, Northeastern University, Pepperdine University, the University of Maryland and many but not all of the University of California campuses.
A number of schools have not jumped on the spring admissions bandwagon. For example, Princeton University does not offer spring semester entry, even for transfers. Similarly, San Diego State University does not accept undergraduate applications for spring admission. Yale University, The Juilliard School and Pennsylvania State University also don't accept students in the spring. That said, there are plenty of schools that do, and the number is only growing.