Does coffee cause dehydration? Actual scientific information on the effects of food and drink is notoriously hard to sift from all the dieting pseudoscience. I know that caffeine is a diuretic, which means it will cause me to lose more water, but given that I'm dosing up on it with water, will I lose more water than I gain? Even if it doesn't for longer drinks, will drinking espresso result in a net loss of water?
Though caffeine's action as a diuretic is hotly debated, recently it seems the scientific consensus is that caffeine in normal doses is a weak diuretic in caffeine-naive individuals only (those without a tolerance), and only in doses exceeding 250-300 mg, or 2-3 cups of coffee. To go even further,


  A profound tolerance to the diuretic and other effects of caffeine
  develops, however, and the actions are much diminished in individuals
  who regularly consume tea or coffee.


And from the same review of the literature: 


  Doses of caffeine equivalent to the amount normally found in standard
  servings of tea, coffee and carbonated soft drinks appear to have no
  diuretic action.


The New York Times exposed this myth in this article:


  Researchers found no significant differences in levels of excreted
  electrolytes or urine volume [between those given caffeine and a placebo].


Here is another good resource for answering these questions in a clear, FAQ-style and features a good list of references on the topic.