This chapter introduces strategies for building new words from existing pieces in the language. The first section introduces the concept of lexical gaps, some of which may be filled by expanding the lexicon using word-formation processes. The second and third sections discuss the two most common word-formation processes, compounding and derivation. The fourth section introduces conversion and semantic extension, which are methods for expanding a lexicon without creating new word forms. The chapter ends with a discussion on borrowing (though borrowing will not be appropriate for every conlang). By the end of this chapter, you will expand the vocabulary of your conlang to include words that have been derived and/or compounded and will consider how you might expand the definitions of existing words to include new meanings.