From my understanding, doctors are licensed in a particular state after completion of their residency, but are they able to freely practice their field of medicine in every state, or can they only practice in their state of license? If not, what additional things are required in order to practice in other states and are they difficult/tedious to obtain? So would an IM residency graduate from NY going to MA for cardiology fellowship, then PA for interventional cardiology fellowship, and finally DC to practice require re-licensing every time the doctor crosses state borders?