Readability formulas are becoming increasingly popular as a tool that writers use to test and revise their material. Used appropriately, a readability formula can provide a quick and easy general measure of how difficult a text may be for its readers. Writers who use a readability formula, however, should do so with caution. Just as an engineer must know the specifications, uses, and limitations of any methodology or tool in the profession, the technical writer needs to understand the origins, uses, and limitations of readability formulas. In this brief paper, I want to give the readers of the IEEE Transactions some background on readability formulas, discuss eight aspects of writing that readability formulas will not help with, and explain why writers should not use readability formulas as a guide to rewriting.