Dear, I live in the US and I don't really understand it either, so you have some company. In fact, I'm not convinced that these government organizations always understand their own roles in things either. I'm sure there are many others on this forum who have a working knowledge of this hierarchy and I'm sure they will enlighten us at some point. As far as I know, yes, the DHHS oversees both the NIH and the CDC. The DHHS is basically the top tier of the cake under the Secretary of Health more or less, which if I'm not mistaken is a 'cabinet' position and answers directly to the president. The NIH in my simple mind seems more generalized, while the CDC is more specialized, but that is not always the case in practical reality. I perceive the NIH to be a little higher than the CDC but that might be because I respect them more today than in the past and certainly more than the CDC. The CDC however, is perceived by most physicians to be the official source of information on all things disease-related. If you quote some statistic from the CDC to your physician, his/her little ears perk up. This, I have found, doesn't happen to the same degree when you quote something from the NIH for some inexplicable reason. I think the CDC has better public relations perhaps. When we watch sci-fi movies about apocalyptic viral outbreaks and such we always see the CDC involved, never the NIH. They are generally perceived by the majority of the American public as scientific geniuses, knowing all there is to know about every disease, while the NIH is more or less perceived as more bureaucratic, vaguely involved in general health issues. Again this does not necessarily represent reality, merely public perception.