Carpet or tile adhesive can seem almost impossible to remove from a concrete or wooden floor, especially when it has been there for a long time. If you try sanding, it will heat up and clog up the sandpaper. If you wire brush it with a wire wheel on your drill, again if you let it heat up, it will just clog up the wire brush. A heat gun can soften most adhesives and allow for some degree of scraping, but it is slow as concrete below will be quite cold and will draw the heat away from the adhesive and bare wood below can just get blacker with melted adhesive. The good news is that most of these adhesives can be softened up by using their original solvent.
New adhesive strippers
Before you go onto the hard way to do this that I explain below -- there is an update on new products to help out. SurfPro from Quebec has one carpet glue remover on the market and now Circa 1850 is bringing one out as well -- so read the lables carefully in the "Paint Stripping" department, you may just find a product specifically for removing carpet adhesive. New "Eco" solvent-free strippers do work but more slowly than ones with solvents. Always cover a stripper with plastic to prevent evaporation and force the chemicals into the old adhesive. Don't work on a very large area at a time for the same reason of not allowing the work to dry out.
Old standard techniques
Water or Solvent based adhesive?
If you can't find the new removers, or find them too expensive, then you will have to go back to the old techniques. First you want to determine if it is a water base or a solvent based adhesive. Take a face cloth and soak it in hot water. Place it over a patch of the adhesive, even cover it with plastic to keep it from drying out. Leave it for about an hour. Try scraping. If at least a little of the top has softened up enough to scrape off with a putty knife, you are in business. Cover as large an area as you can reach with hot water and something to avoid evaporation. You can dam up the water over concrete but be careful over hardwood as the water will flow between the cracks. You can simply keep rags soaked. You will soon learn just how long it takes for each patch to soften up. This is a slow process but the more patience you have the less elbow grease you will need.
Using Heat
If there is a clear varnish on a wooden hardwood floor under that glue, you can usually successfully soften the varnish with a heat gun, even if your don't soften the glue. The soft varnish will permit you to easily scrape off the glue on top of it. In this case, sanding lightly and refinishing could give you a nice hardwood floor because the varnish kept the black adhesive from staining the wood.
Using solvents
If the neither water nor heat affect it, try the same technique with paint thinner. If you have to go this route, turn off all heating devices, all pilot lights and assure yourself some good cross ventilation. Put plastic over the soaking rags to prevent evaporation, to force the solvents down into the adhesive and to have less fumes to deal with. Work in a relatively small area at a time. This should soften up just about any adhesives that won't respond to water. Hang the rags out to dry or keep them in a closed metal container to avoid the possibility of spontaneous combustion and fire. If you can find the new gell like glue strippers mentioned above, they will be less volitle and more effective than paint thinners.