Sadly, Oprah's beloved cocker spaniel Sophie died on March 10, 2008. "Sophie gave me 13 years of pure unconditional love," Oprah says. "She was a true love in my life. In fact, she's been one of the greatest reasons for me to be a kinder, gentler person."

Just before Sophie passed away, Oprah—and thousands of drivers—saw a billboard just off the Kennedy Expressway in Chicago that read, "Oprah: Do a show on puppy mills. The dogs need you."

SEND US: Pictures Of Your Rescued Pets

That billboard was the idea of Bill Smith, who founded Main Line Animal Rescue in 1998 to stop unnecessary killings of dogs at puppy mills. The man behind the billboard is Bill Smith, founder of Main Line Animal Rescue. Every year, Bill and several volunteers rescue hundreds of abused, unwanted or abandoned animals, rehabilitate them and adopt them out to families. Many of the animals Bill rescues come from puppy mills, which he describes as places where bad breeders care more about the profit they make from puppies than the health or welfare of the animals. He says the dogs in puppy mills face deplorable conditions including inbreeding, minimal or no vet care, limited shelter and overcrowded cages.

View a preview of Lisa Ling's report by clicking on the video below.

"We were so frustrated … and no one seemed to be helping them," Bill tells Oprah. "I know that you're a huge animal lover. I just thought that you would be able to spread the word and educate a lot of people."

The billboard certainly worked. "It is my belief that when you actually see this, America, with your own eyes," Oprah says, "that you are not going stand for it."

To see what goes on at puppy mills, Lisa Ling brings a hidden camera and joins Bill on a tour of puppy mills in Pennsylvania. According to The Humane Society of the United States, there may be as many as 10,000 puppy mills operating across the United States.

Lisa and Bill also visit two pet stores. Many of the dogs born in puppy mills end up in pet stores or sold on the Internet. Lisa and Bill want to trace puppies' paperwork back to specific puppy mills. At the first store, they find a cockapoo puppy, and at the second, Labrador retriever puppies.

While the owner of a store they visit denies his dogs come from puppy mills, Bill says most at pet stores do.

"The fact is, what they're doing is not illegal," Lisa says. "But the objective I think here is to raise awareness. People go into the pet stores, and they see these cute little puppies not knowing where they come from and what they go through."

Lisa and Bill spend the next two days tracing the pet store puppies back to their breeders. Bill has fostered relationships with hundreds of breeders across Pennsylvania. He asks them to give him the dogs they no longer want or, worse, the ones they're planning to kill. Bill says these breeders most often want to get rid of older females and younger males. Breeders only need one or two male dogs to breed with every 20 fertile females, so young female dogs are valuable in puppy mills.

Lisa and Bill head to the breeder where the black cockapoo was born. The breeder tells them its mother is one of the dogs in cages outside. Lisa and Bill then followed the breeder into a building full of cramped wooden crates stuffed with dogs. "The spaces are so small, the mothers are stepping all over their babies," Lisa says. Bill says he thinks many of these dogs have never walked in the grass though they live on a 60-acre property.

Visiting the home of the Labrador retriever puppies, Lisa and Bill find the lab puppies' mother in a small, mud-filled pen. Her teats are swollen—Bill says this is evidence she's had many litters of puppies.

Lisa and Bill continue on to see other nearby puppy mills. In one, they find 30 to 40 dogs in wire-floored rabbit cages stacked to the ceiling in a room that reeks of urine and feces.

Before they go to one breeder's property, Bill warns Lisa about what she may see. "It's probably the worst place I've ever been to in my life. He has dogs running on wheels in fan casings," Bill says. The contraptions look like big gerbil wheels. "[The breeder] claims that it's good for them because they get exercise," Bill says.

That breeder wouldn't let them see his dogs, but Lisa and Bill saw two dead animals on the property.

At a third breeder's facility, Lisa and Bill see outdoor cages that house scores of Pomeranians. Bill tells Lisa that not only have those dogs probably never been out of those small cages, it's likely that they remain outside even in bad weather and could die from exposure.

After finishing their rounds collecting unwanted dogs from puppy mills, Bill and Main Line Animal Rescue have dozens of dogs. Lisa says seeing the way the animals live was haunting. "While I feel relieved about those 39, there are hundreds that we just saw that will remain in cages for the rest of their lives," Lisa says.

While what they do is difficult to witness, Bill explains that puppy mills aren't against the law in Pennsylvania. It's even legal to shoot dogs there, as long as it's not done maliciously. Bill says there are good breeders who actually call him and give Main Line Animal Rescue the opportunity to save dogs. "It's always amazing to me when I go out to pick up a dog, and they've had the dog eight or nine years, and it doesn't have a name," he says. "It's never been out of the hutch, it doesn't know how to walk, and I have to carry it to the car. It's heartbreaking."

Lisa says one reason puppy mill owners keep their dogs in such conditions is cultural. "They don't regard dogs in the same way that others may. They believe man is to dominate animals," she says. "A number of them didn't seem like they realized that what they were doing was inhumane because in their culture … that is what they're supposed to be. And the fact is, there is a market for puppies in America."

Bill says he's asked the breeders who own puppy mills why they treat the dogs so badly. "They think that we're fools when we pick the dogs up," he says. "I just went back to one of the mills, and they were asking me about the cocker spaniel we pulled out. … And I said, 'Well, she's fine. She's walking around the house and everything.' And he said, 'You let that dog walk around the house, where the people in your family live?' And I said, 'Yes, we do.' He just couldn't get over it. It's a different mentality. [Dogs] are considered agricultural products. They're like an ear of corn."

The morning after their rescue mission, Lisa returned to Main Line Animal Rescue to see what the team of volunteers was doing for the new dogs—and the dozens of other neglected or abandoned dogs, cats and rabbits in their care.

Each rescued dog has an appointment with a veterinarian, which is often the first medical care they've gotten in their lives. They also experience two more firsts—their first bath and first grooming. "[We] try to get them cleaned up because they smell really, really horrible," Bill says.

For some dogs, the medical care is urgent. Some have mammary tumors caused by years of nursing and overbreeding. Bill estimates that one cocker spaniel could have given birth to as many as 140 puppies.

Before they were rescued, many of these dogs spent their entire lives in wire cages and had trouble walking on the ground once they were out. Some had their vocal chords damaged by a pipe in order to keep them from barking. Others were completely filthy, with their coats overgrown and soaked in urine.

Despite the inhumane conditions the dogs lived in before their rescue, Bill says they usually start to show signs of rehabilitation in about two weeks. The first indication they're coming around is when their tails start wagging, he says.

Bill says Main Line Animal Rescue has rescued approximately 7,000 to 8,000 animals—about 5,000 of those from puppy mills. Their mission is to ultimately place each rescued animal in a loving family, and they are remarkably successful. "We have one of the highest placement rates in the country," Bill says. "We place about 99 percent of the animals we take in because we have a really good trainer who comes and works with us. We actually have something called a 'shy dog' class where we do massage therapy on them and everything else. It's great."

Weeks later, the rescued dogs are steadily making progress. "They're doing extremely well," Bill says.

Even if they've been living in one place for years, most of the dogs that come from puppy mills have never been named. That's one of the first things Bill and the volunteers at Main Line Animal Rescue do.

Bill says several of the dogs he and Lisa rescued were named in honor of their appearance on The Oprah Winfrey Show. "We have a 'Gayle' and we have an 'Oprah' and we have a 'Stedman,'" he says.

Over the years, Bill says he's definitely taken some of his work home with him—he has 10 dogs! At one point, Bill says he had 12, a bunch he affectionately called the "dirty dozen."

Bill says he usually keeps the dogs who most people would consider "unadoptable." "I have three blind dogs and two dogs with back problems and one dog that was feral in a junkyard," he says.

In January 2007, Bill rescued a Maltese-poodle mix from a breeder. The emaciated puppy had little hair and was close to death. Bill couldn't leave him behind and adopted the little guy, naming him Shrimp.

Today, Shrimp is a happy and healthy member of Bill's pack—and even wears a red bow in his full mane of hair. "Boy, what a makeover!" Oprah says.

If you are thinking about adopting a new pet, make your first stop the local shelter or animal rescue office. "You can find any kind of dog you want, any age you want, at a shelter or rescue," Oprah says.

Sometimes, what you see isn't always what you get when it comes to that doggie in the window of a pet shop. "You can be deceived when you see these cute puppies in the stores," Lisa says.

If you are looking for a specific breed of dog, Bill says you're likely to find one at a rescue. "A third to 50 percent of the animals in shelters, depending on the location, are purebred dogs," Bill says. "The rest are great designer mixes." Bill says there are also breed-specific rescues throughout the country.

For people who would still like to use a breeder, Bill urges you to find one that is responsible. Quality breeders care about the quality of life their dogs will have when they go to their new homes and will often interview you about your home. "One of the things that makes them a responsible breeder is they take their dogs back even if it's six or seven years later," Bill says.