Chocolate contains theobromine, a substance similar to caffeine.  Dogs metabolize theobromine very slowly, so toxic amounts may accumulate from ingesting relatively small amounts of chocolate.
The Handbook of Poisoning in Cats and Dogs by Alexander Campbell and Michael Chapman suggest the fatal doses of theobromine for dogs is in the range of 90-250mg/kg body weight. To put it into relative terms a small bar of plain chocolate (100-150g) will be enough to kill your average sized Yorkie. A medium sized bar (200g) an average sized Spaniel and a large bar (400g) an average sized Labrador. In some susceptible individuals it may be significantly less.
Baking chocolate contains more theobromine per ounce than semisweet chocolate, which, in turn, contains more theobromine than milk chocolate.
The sooner treatment for chocolate poisoning occurs, the better the chance for recovery. If you witness your dog eating chocolate or if it shows any signs of chocolate toxicity (anxiety, pacing, hyperexcitability, excessive thirst and urination, vomiting, or seizures): call your vet immediately and tell him or her what type of chocolate your dog ate, how much you think it ate, how long ago it indulged itself, and how much your dog weighs.
There is no antidote. If treatment begins within three hours of ingestion, the vet will induce vomiting to empty the stomach and prevent the toxins from entering the circulatory system. This may be followed by an adsorbent charcoal mixture into the stomach. This will help stop further absorption of the theobromine and reduce the signs that subsequently develop.
If more than a couple of hours have passed, the toxin will        already be circulating in your dog's system, so your vet will  provide supportive therapy (such as intravenous fluids and drugs to control hyperexcitability) while your dog's body works to detoxify itself.
For more information call Animal Poison Control (they will charge an initial fee for a consult) 1-800-548-2423.