Hardwood floors over concrete slab? What to Consider for proper/level install?

I'm thinking about putting hardwood floors in my den which is on a concrete slab, no basement or sub-floor beneath it. Right now it's just carpeted. My question is what do I need to consider/think about or ask the contractor to ensure the floors get installed correctly and level? For example, I know for a fact the floor is not that level, will it need to be re-surfaced and leveled first somehow (just my uneducated guess) or is there something they use when installing hardwood floors over concrete, or is it just good craftmanship I need to look for, etc? Is it recommended or common that hardwood floors can be installed over that kind of surface. Many thanks to all. Regards.
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Atari26004Fun wrote:

Do a google for hardwood floor manufacturer's association -- they have a very informative web site although I don't recall the url otomh...
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Thanks..... :)
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http://www.hardwoodinfo.com/display_article.asp?ID00
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Hardwood is not recommended over concrete unless a vapor barrier and insulations is fir installed, This means adding 2 x 4 sleepers. Engineered wood, however, can be done with the proper barrier.
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Atari26004Fun wrote:

Not level so much as flat.
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