Dogs should be at least 6 months of age and weigh at least 6-8 pounds.  The collar length can be trimmed to fit a neck size from 8-26 inches. Cats should be at least 1 year of age and weigh at least 3-5 pounds.  The collar length can be trimmed to fit neck sizes down to about 6 inches.  The 'cat' version can also be used on smaller dog breeds or very timid dogs, since the vibration isn’t quite as strong.