80 years have passed since the publication of A. S. Hornby et al.’s Idiomatic and syntactic English dictionary (1942), which was reprinted a few years later by Oxford University Press as A learner’s dictionary of current English (1948). Hornby et al.’s publication firmly established a new and distinct genre of dictionary that has been at the forefront of lexicographic innovation during the past decades. This paper examines the historical development of selected key features of monolingual English learner’s dictionaries, focussing on major innovations and their significance for the users. It concentrates on areas such as defining vocabularies and definitions, grammar and usage, example sentences and the use of corpora, and also briefly touches upon electronic learner’s dictionaries.