Our new kitty Sheba is such a wild adventurer, and is strictly indoor. I'm positive she would catch a collar someplace during her kooky gymnastics and strangle herself. I think collars are great for dogs, but cats are more like toddlers in terms of getting into weird situations- collars on toddlers would be a safety hazard so that's how I see it. Sheba came to us from the shelter already microchipped...but all our future kitties will get a chip too.