My cats have fleas!  Or, at least, they had fleas.  I have used over-the-counter, less expensive items (flea powders, flea collars, etc.) up to and including other topical products like this one, and nothing worked.  The flea collars were less than worthless - they actually caused patches of fur to fall out on at least one cat.  The powder was unwieldy, and the other topical applications (save one) did not work at all.