It is hard to believe for many people that something as yummy as chocolate, which is generally so harmless to people, can be quite toxic, and indeed lethal to dogs. The toxic agent in chocolate is theobromine, and it can cause both neurological and cardiac toxicity to dogs when ingested in sufficient quantities.
Theobromine is most concentrated in raw cocoa bean, and is at much higher levels in pure dark chocolate, and in lower levels in milk chocolate (there is none in white chocolate). Toxic doses will depend on the type of chocolate and the size of the dog, but for a 5kg small breed dog, as little as 10g of pure dark cooking chocolate can cause toxicity.
There is no antidote, and veterinary supportive care is critical to treat toxicity, so if in doubt, contact your vet if you think your dog has eaten chocolate. Christmas and Easter are often times when we see more cases, as dogs will sniff out chocolate in presents under the tree, or from post Easter chocolate feasting.