Learning to express doubt and certainty in English is a complex task, but an important one since epistemic devices also function pragmatically as politeness markers. Those preparing materials or writing textbooks for the ESL learner must decide which epistemic devices to include. Information on the range and frequency of such devices in naturally occurring speech and writing can provide a basis for the necessary selection. This paper provides some data on the relative frequencies of a wide variety of lexical items expressing doubt and certainty in written and spoken corpuses, and uses the data to evaluate the adequacy of some well-known ESL textbooks.