It is assumed by many teachers and materials writers that it is important to teach at all levels the forms and meanings of derivational prefixes and suffixes, as an important aid to learning new words in English. However, corpus evidence suggests that knowledge of the meanings of such components occurring within the most common 5,000 words in English--a rough upper target vocabulary level for intermediate-level (B1) students--does not, in most cases, help learners at this level to understand the words of which they form a part. It is concluded that probably only a relatively small number of prefixes and suffixes are worth teaching in courses for intermediate classes.