Research has consistently shown that a few thousand word families can be enough to understand most written or spoken text (e.g. Nation, 2006;Van Zeeland & Schmitt, 2013). The implication is that these frequent items need to be learned -and to be learned thoroughly. However, diminishing returns set in, and accessible tools are required for the many tens of thousands of other items. Dictionaries are among the most widely-used tools for foreign and second (L2) language learning, and can help with both frequent and infrequent items. This chapter begins with a discussion of dictionaries themselves in relation to L2 use, and the main issues affecting their development over the last few decades in particular. These include the increasing use of empirical data, especially in the form of corpora, and the appearance of monolingual learner dictionaries in addition to bilingual and other dictionary types. It then moves on to a discussion of research into how dictionaries are used by non-native speakers (NNSs), especially as a reference resource for encoding and decoding, but also their impact on language learning itself.