Language use is regulated in part by pragmatic principles. Speakers adhere to such principles in order to make language work for them effectively in conveying their intentions to others. Two of these principles are Conventionality and Contrast. In this chapter, I explore some of the consequences of these principles for the lexicon. These two principles are central to language use because they (a) govern use of the well-established lexicon, the stock of words accrued to the language over time, and (b) place constraints on what speakers can do by way of coining new words on particular occasions. I begin with a brief account of Conventionality and Contrast, and how they act together in governing lexical usage, and then look at where we can see these principles at work, both in everyday adult speech and in child acquisition.