This chapter examines the peculiarities of monolingual learners' dictionaries (MLDs), both in print and electronic form. Learners' dictionaries show various unique features and have – also due to their sales and innovativeness – become a firmly established and distinct genre within English lexicography. Perhaps most strikingly, MLDs use a limited and simplified vocabulary for definitions, known as "defining vocabulary". MLDs assume little about the learners' linguistic background except for certain proficiency with English. The learners' native language may be closely related to English, partially cognate, or even entirely different structurally and conceptually. In contrast to many dictionaries for native speakers, learners' dictionaries indicate phonetic transcriptions by means of the International Phonetic Alphabet (IPA). This tradition dates back to Advanced Learner's Dictionary of Current English (ALD1) and are well justified, as most foreign learners of English become familiar with the IPA in the course of their language studies anyway.