Board certification (thru ABMS or AOA) only matters for hospital privileges, insurance reimbursements, faculty appointment and medical-legal protection (civil suit as well as pt complaints). To be eligible to take ABMS/AOA board certification exams, you must complete their respective residency/fellowship training. However, technically speaking, you can practice whatever kind of medicine after you are licensed by your state's medical board (i.e. after 1-2 years of PG training for US DO/MD) as long as you feel comfortable/competent to do so even without board certification or additional residency training. However, do not expect to be doing it in someone else's hospital/facilities and don't expect to get paid for it or fully paid for it by the insurance companies and more importantly hire the best d**n lawyer in your state because you will need his/her services soon.
I think I need to clear up some things since the medical students are trying to answer questions about a level they have not achieved yet.
Medical-legal protection comes down to how blantantly bad a doctor you are and how good your malpractice insurance is. BC is not involved.
During medical school you take Step I/StepII/Step IIPE (COMLEX, USMLE, or both)
Most medical schools now will not graduate you without passing of these three
I you fail any of these steps more than 3 times you will be hard pressed finding a residency who will take you and many states WILL NOT LICENSE YOU. Texas will not let you do residency there if you have failed 3 times.
You take step III during your intern year and need that in order to apply for a license in that state. You finish residency and take the Board Certification exam the third year
To say that you don't need to finish residency or
only need board certification for hospital priviliges is COMPLETELY FALSE.
If you plan on getting hired anywhere, whether its locum tenens, or small medical practice, large medical practice, or hospital employee. You would be hard pressed to get hired. I have a few friends who didn't finish residency who work urgent care clinic but that really limits your job opportunities. I have worked approx 20 different sites in 5 states doing locums work and every single site required board certification. It's more important in the job market than is being expressed on this thread.
You apply for Fellowship after you finish residency. Each have their own criteria and you to look at each individual program to see what is necessary for consideration. Fellowship looks at boards scores but they also depend on a letter from your residency program director and your rotation evaluations in residency, competency, and your overall success as a resident physician. Do you "play well with others", etc. Again, just like every hoop on the journey comes down to the total package.