The Marco Polo tag only weights 0.8 oz., so it is light enough for most any pet to carry. It does require a minimum collar length of about 7 inches, if you have a very small cat or dog then measuring the length of their collar is your best guide. The cloth holder that we supply attaches to the collar or harness that you pet already wears, from 3/8” wide up to 2 ½” wide – there is no upper size limit to the animals that can wear Marco Polo! If you have a cat that currently doesn’t wear a collar then it is best to get them used to a collar first and then add the tracking tag.