In Chapter 4, we examine the supply of dogs to the U.S. market. We first explain two potential sources of social inefficiency in the market: information asymmetry and negative externality. We then explore the heterogeneous nature of the sources of supply. Based on organizational form, we distinguish between commercial and so-called backyard breeders. Based on the ethics of practice, we distinguish between ethical breeders who provide high-quality care and unethical breeders who do not. We relate our six resulting categories of supply, including puppy mills and shelters, to the information asymmetry, negative externality, and a third potential market failure, the public good nature of the problem of protecting dogs from cruelty. Finally, we argue that serious market failures provide justification for government intervention to increase efficiency. However, the current patchwork of regulations and lack of resources invested in enforcement allow puppy mills and black-market breeders to impose costs on others and harm dogs.