That will depend on several factors. If you have employer-sponsored healthcare, are actively working and your employer group has more than 20 employees, the group plan will be primary. In this case, you will be able to delay enrolling in Part B until your situation changes. You will want to make sure that you sign up for Part B in a timely manner if you are retiring or losing your group coverage to avoid Late Enrollment Penalties. If you do not have employer-sponsored healthcare or your employer group has less than 20 employees you will want to enroll in Part B upon aging into Medicare.