This article offers a critical review of research on the use of glossing and its contribution to second language (L2) vocabulary acquisition. Discussion topics include the complexity of estimating the effectiveness of glossing relative to reading non-glossed texts, the quest for optimal implementations of glossing, issues of ecological validity, and ambiguity around the nature of vocabulary learning from glosses. The general conclusion is that, despite the substantial number of research studies on this subject, many questions remain to which only tentative and provisional answers are currently available. This is partly owing to the wide diversity in research designs across studies and the lack of transparency of many research reports. Suggestions are made for further research on glossing with a view to enabling future systematic reviews to produce more nuanced answers and more informed recommendations for the design of L2 reading materials.