Chocolate itself is not poisonous to dogs, but it contains theobromine. Theobromine is a compound that can act differently in different dogs. Theobromine can bring on epileptic seizures in dogs that are prone to them. It can also cause cardiac irregularity, and in severe cases can bring on a myocardial infarction, which can kill the dog. In some cases, it can cause internal bleeding, which left untreated can quietly kill them over several days.
Walnuts are especially dangerous because they commonly carry a fungus that causes seizures in dogs.
Since chocolate and walnuts are a common ingredient in candies and foods around the holidays, make sure you tell your family members not to give the dogs any table scraps or holiday cookies. Explain to them your reasons why this can actually be harmful to your dog and is not a treat. If your dog has ingested any of these items, immediately call your vet or check with the emergency room of your local hospital and ask for the number of the local Poison Control Center. You can also call the National Animal Poison Control Center, which offers three fee-based hotline numbers. Be sure not to induce vomiting unless told to do so after consulting with your vet or an animal poison control center.