De-thatching and overseeding?

I enjoyed your article on lawn care.  Could you share your views on de-thatching and overseeding?  I recently moved into 12-year old house with a fairly well kept lawn, although we have some weeds and a few brown spots.  I am preparing to fertilize and notice heavy thatch beneath the grass.  Is it necessary to remove it?  Could the lawn benefit from overseeding even in the areas where it's growing well?  Thahks for any advice you can provide.

I think that thatch comes from shallow watering.  I could be wrong. 

I think I would start a better watering program and ignore the thatch - it will eventually go away.  Mechanical de-thatching takes work, $$$ and makes your lawn look like hell until the grass grows back. 

There are times where overseeding can help, BUT!  I would first want to find out why things aren't growing in that spot right now.  What's the use to toss grass seed on a patch of cement?  If the lawn is thin in a few places, add some compost to those spots.  It will improve the soil and when the existing grass gets healthier, it will send out rhizomes and produce more grass!