The shelter that I volunteer at and adopted both Doogie and Fawnie at, microchipped both of them and also Sydney, my foster mama. None of them had a problem with it. It was ME who had a problem with looking at the HUGE needle. Like emily_the_spoiled said, the chip itself is the size of a grain of rice. I think it's well worth it in case your cat gets out, falls out of a window (uh, ELI!!!), etc.