The domestic cat is a charismatic and pervasive non-native predator whose population in the United States is an estimated 114-188 million. When permitted to roam outdoors, cats pose serious risks to the health and welfare of wildlife, people, and the cats themselves. Despite overwhelming evidence to support keeping cats indoors, many municipalities continue to endorse the maintenance of cats outdoors through Trap-Neuter-Release at the behest of feral cat advocacy programs and their vocal grassroots supporters. The American Bird Conservancy (ABC) Cats Indoors program strives to raise awareness about the negative impacts of outdoor cats, to educate policy makers and the public, and to promote responsible pet ownership. We review the scientific evidence that necessitates keeping cats from roaming outdoors and how ABC’s Cats Indoors program is working to protect wildlife, cats, and people.