Chocolate is one of the most common causes of poisoning in dogs.
It contains the stimulant 'theobromine' which cannot be metabolised by dogs unlike humans.
Puppies (inquisitiveness) and small breeds (relatively small body size) are more at risk.

Cocoa powder and baking chocolate are the most toxic, followed by semi-sweet, dark, milk and white chocolate in that order.

Symptoms include: vomiting, diarrhoea, restlessness, hyper-excitation, increased urination, panting, and in serious cases coma and death.

You should seek immediate veterinary help if you suspect/know your dog has eaten chocolate. The vet may need to make the dog vomit it back up, give it medication to absorb the chocolate in the gut and in some instances keep the dog in hospital for monitoring and further treatment.