"SubI" is short for "subinternship" and it is exactly what its name suggests...you act just below an intern, with the only difference being that the intern has a medical school diploma. The subinternship is the time for you to learn the things that you will need to know to function as a resident that may or may not have been taught to you in other rotations, such as writing orders, discharge instructions, prescriptions. You generally will do procedures that interns and residents do such as central venous/arterial access, para-/thoracentesis, intubation, LP's etc. The difference between subI and other medical student rotations is that for other rotations, if you do some of the things I mentioned, you impress residents with your work ethic and enthusiasm. In your subI it is expected of you. If you dont write your orders, it is likely that they will not get done, unless your senior resident does it for you. If your work doesnt get done, it reflects poorly on the senior resident and you are likely to get a poor evaluation. Generally the expectations of you are the same as the intern; eg, you take call redularly, you carry multiple patients, you must know all aspects of your patient's care. Generally, the senior resident will be the only other person on the team who follows your patient. The intern/junior resident/medical students will not follow your patient so no one can bail you out if you dont know something about your pt when the attending asks. If you do a subI in surgery or OB, you may be first assist on some of the easier cases. (I know a subI who did an entire C-section on a patient while the attending watched.) It is supposed to be the most labor intenesive part of 4th year. Others have recommended doing a subI at your top choice for residency, because since it is such a tough 4 weeks, it shows that you are really interested in their program. Most people are aware of this so it is difficult to get a subI at competitive programs. On the other hand, it can easily expose weaknesses in your work ethic if lacking.