If you have calluses on the bottom of your feet this indicates that you have a problem with your foot biomechanics.  A biomechanical problem means that there is a problem in the way in which the many joints, muscles, ligaments and tendons of the foot and ankle are functioning.  As you walk and run your foot is subjected to tremendously high loads.  When all the muscles, ligaments, tendons and muscles are working as they should the load is distributed appropriately and calluses do not develop.  If the is a problem with the biomechanics, the load is not properly distributed and calluses develop in the areas that take the higher loads. 

Your body adapts to the physical demands that are placed upon it.  Calluses form where there is too much pressure or friction on your foot.  Your body responds to excess pressure on the bottom of the foot by thickening the skin (callus) in the high pressure area.  The callus develops to protect that area. 

Although the body develops the calluses for protection, if untreated the calluses can do harm.  Calluses that are not treated will become painful.  The excess thickness of skin may itself cause pressure and pain in the area.  The body will start viewing the callus as a foreign body and mount an inflammatory reaction.  Calluses will often cause a tear in the skin where it interfaces with the surrounding softer skin.  These cuts can lead to infection.  This is a particular problem for diabetics.  People with diabetes often have a difficult time healing cuts in the skin and fighting infections.  If a diabetic develops an infection in their foot this puts them a great risk for amputation.  It is essential that diabetics protect their feet to prevent calluses from forming and if a callus develops they must seek attention immediately. 

If the calluses are large and at risk for causing a skin tear they should be removed by a podiatrist.  You should avoid using over-the counter products to remove the callus.  These products often contain acids that can damage the healthy skin.  While waiting for your appointment with a medical professional you can reduce the callus by soaking your foot in warm water and carefully using a pumice stone to gently smooth the callus.

To prevent calluses from reforming once removed you need to correct the cause.  If the cause was ill-fitting shoes you should be fitted by a professional shoe fitter.  If caused by a biomechanical problem such as flat feet, overloaded joints, poorly functioning ligaments and tendons, a custom foot orthotic may be necessary to correct the problem.