Researchers on the vocabulary growth of native speakers of English, usually distinguish three main ways in which a learner’s vocabulary increases – through being taught or deliberately learning new words, through learning new words by meeting them in context, and through recognising and building new words by gaining control of the prefixes and suffixes and other word building devices. In this chapter we look at the extent to which word building affects vocabulary size, the psychological reality of the relationship between inflected and derived words and their stem form, and the teaching and learning options for gaining control of English word-building processes. There are two related but distinguishable reasons for focusing on word parts. First, prefixes and stems can work as mnemonic devices to help learners remember new words by relating them to the meanings of the known parts that they contain. Second, knowledge of prefixes and suffixes can help learners see the relationship between word family members where one or more of the members is already known. The chapter contains lists of useful word parts and describes a strategy for using word parts to help remember words.