It's typically common knowledge that chocolate is toxic to pets, but how much and what kind makes a big difference on how toxic.
The toxic compound in chocolate is Theobromine which is a methylxanthine. Methylxanthines cause an increase in circulating catecholamines (such as epinephrine) and also cause problems with the uptake and storage of calcium. And of course most chocolate products contain a large amount of fat which further disrupts the GI tract. Caffeine is also present in most cocoas and can cause similar signs as theobromine.
So what are the signs? At low to moderate doses chocolate ingestions causes vomiting, diarrhea, restlessness and hyperexcitability and increased heart rate. At high doses irregular heart beats and seizures can be present.
Now, the big question is how much is too much? First we need to evaluate which products are the most dangerous. The following list ranks the most common kinds of chocolate from the lowest (least toxic) amount of theobromine to the highest (most toxic).
Dry Cocoa Powder
So to put it in perspective a regular sized Hershey bar is 1.5 ounces. For a 30 lb dog to consume the whole bar would not likely even cause mild signs. But for a 10 lb dog the same bar will just put you into the mildly toxic range. Three Hershey bars however would put said 10 lb dog into the seizure level of chocolate exposure. For another example suppose the 30 lb dog ate 1.5 ounces of baking chocolate-this puts them into the mid range of toxicity where cardiac signs start to show up. If our 10 lb dog eats 1.5 ounces of baking chocolate they are way past the seizure level! Now for our big dogs our there could potentially eat 8 Hershey bars before reaching the mildly toxic mark! So hopefully that gives you some idea of the differences based on size and product type. Also realize that no matter how small your dog is, don't panic if they eat a Hershey Kiss or three M&Ms-they will be ok!
So, what if they do eat a suspected toxic amount of chocolate. Try to evaluate how much they ate and what kind, then call your vet. If they think your pet is in danger of becoming ill they will likely have you come in to induce vomiting if the exposure has been in the last four hours. They may also administer activated charcoal to absorb some of the toxin from the GI tract. Additional therapy includes IV fluids, EKG monitoring, and seizure control. Most dogs fully recover although some may develop pancreatitis as a consequence of all the fat ingested!