Learners of English might be familiar with several online monolingual dictionaries that are not necessarily the best choices for the English as Second/Foreign Language (ESL/EFL) context. Although these monolingual online dictionaries contain definitions, pronunciation guides, and other elements normally found in general-use dictionaries, they are compiled with native or near-native speakers of English in mind, not for learners of English. English learners tend to gravitate to these dictionaries because they are seemingly unaware of other dictionaries that are specifically designed for them. Research on dictionary use in second language teaching indicates learners of English gain much when they become familiar with dictionary features. For this reason, ESL/EFL teachers should learn about and introduce the several excellent online dictionaries with features especially designed for learners of English. These online dictionaries promote strategic and effective word learning, but it is critical for learners to be trained by teachers who are familiar with proper dictionary use so that the online dictionaries are used to maximum benefit in the classroom. The best online learner dictionaries include: (1) a corpus-based compilation of words; (2) word frequency data; (3) collocation guides; (4) authentic examples of how words are used; and (5) topical vocabulary from different disciplines. This article helps teachers become aware of these important features and also provides suggestions on how to integrate dictionary-related exercises into their ESL/EFL lesson plans. (Contains 1 table.)