Tuckpointing:

Age, weather and freeze/thaw cycles are all contributors to the deterioration of exterior masonry chimneys.  
Tuckpointing is a procedure whereby some or all of the chimney's mortar joints are ground out to a minimum depth and new mortar is applied.  

             Before...                             After
                             Click on photos to enlarge

Chimney Rebuilds:

When the extent of damage is beyond the scope of tuckpointing, a chimney rebuild is the recommended procedure.

                   Before...                           After