Before hiring arborists, check to ensure that they are certified. While this will not guarantee the desired outcome, at least you are starting the project with someone "a cut above" the sort of jack-of-all trades who may offer to do the work for less money -- but who may only exacerbate the problem. Certified arborists are accredited by the International Society of Arboriculture.
So what do arborists do? Homeowners call upon these professionals to undertake all kinds of work pertaining to trees, ranging from establishing specimens properly in the landscape to tree-care issues that may crop up over the lifespan of a specimen (pruning, dealing with diseases and pests, etc.) to removing a tree that poses a hazard to your home or landscaping.