My 3 cats are indoor cats and don't wear collars. They were each equipped with a small microchip under the skin on their backs before we adopted them at the humane society when they were spayed and neutered, it's about the size of a rice grain. You just fill out the paperwork that comes with the chip and get them registered. If one of them gets out, a satellite can pinpoint their exact location and they can be rescued and we don't have to worry about them choking themselves to death with their collar on a piece of furniture or anything like that. Collars seem like they would be uncomfortable to me. I want my cats to be as free as they can be. Hope that is helpful!