Recently there has been some debate about the appropriacy of different lexical units in pedagogy and research (e.g., Brown et al., 2020; Dang & Webb, 2016a; Kremmel, 2016; Laufer & Cobb, 2020; McLean, 2018; Nation, 2016; Nation & Webb, 2011; Vilkaite-Lozdiene & Schmitt, 2020). The lexical unit (word types, lemmas, flemmas, word families) needs to be considered when developing wordlists, vocabulary tests, and vocabulary learning programs. It is also central to the lexical profiles of text and corpora, which indicate the vocabulary learning targets associated with understanding different types of discourse. Perhaps most importantly, the lexical unit of words found in vocabulary learning resources such as word lists and tests may affect their pedagogical value. The aim of this article is to highlight aspects of research and pedagogy that are affected by lexical units and describe issues that should be considered when operationalizing words in studies of vocabulary and learning resources.