Import cannot be resolved even after adding to build path and cleaning

I'm having a rough time getting the Apache HttpClient jar libraries imported in the Eclipse ADT. I downloaded the jars. I added them to the project internally and to my build path, and cleaned the project. I kept getting the "import org.apache.commons.httpclient cannot be resolved" message. I tried adding them as an external jar, cleaning the project again, and closing and reopening Eclipse for good measure. No luck. It still has no idea that the jars are there. I feel like there's something basic I'm not doing, but I can't figure out what it is. It seems that most of the time that someone has a problem like this, what fixes it is doing one of the things that I've already done and hasn't worked.