Beginning PhD in related field without subject area expertise, will that be a problem? I just finished an undergraduate degree in psychology, with a focus in biological sciences and pre-medicine courses. In looking at my options for graduate school, I've taken an interest in biomedical engineering; tissue engineering, medical devices, medical imaging, etc. My biology is pretty solid, but I'm not much of an engineer, aside from a few math courses. My question is, are students entering a PhD program typically very well-versed in the field, or is there a lot of "learning on the job"?
While shan23's answer is good, it is also somewhat incomplete. There are a few things to watch out for when you're coming in to a field from another department:


You will need to keep in mind the qualifying procedures for your new department. Will they expect you to pass exams in undergraduate coursework in the new discipline? If so, then you'll need to do a lot more "catch-up" work early on to make up for the potential shortfall.
Unless the new area is an interdisciplinary one—such as biomedical engineering—they're probably going to want to see some track record in the area. You're going to find it a lot easier to move into biomedical engineering from mechanical engineering than from economics.
You may find it helpful to try to find a position as a lab assistant or something similar to this in the new field before you try to start the graduate coursework. However, this is by no means required. (But it would help to prove the "dedication" aspect, which is what you'd need to convince a graduate school admissions committee about in order to have a successful application).