When looking fora new dog, it's important to find a good breeder. That's the advice from experts who say that puppy mills— mass dog-breeding facilities that sell puppies through pet stores and over the Internet—often treat their animals cruelly and inhumanely.

According to The Humane Society of the United States, puppy mills frequently house dogs in shockingly poor conditions, particularly for the "breeding stock" animals who are caged and continually bred for years without human companionship and then killed, abandoned or sold to another mill after their fertility wanes.

An estimated 10,000 of the mills operate in the U.S., forming a billion-dollar industry. The mills sell dogs through many pet stores, over the Internet and sometimes through newspaper ads. Opponents of the mills say the best way to put them out of business is to not buy the puppies they sell.

Instead, they suggest visiting animal shelters, purebred rescue groups or a responsible breeder. Nationwide, one out of every four dogs in an animal shelter is a purebred, and an estimated 6 to 8 million animals enter shelters annually, with only about half of them being adopted.