In this paper the concept of “lexical unit” as proposed by Cruse (1986) is introduced in the context of L2 vocabulary learning to replace the rather vague notion of “word.” In the first part of the paper, this concept is defined in terms of lexical semantics and then applied to the L2 learning task. The second part describes two experimental studies in which the learning of different types of lexical units is examined. In the first study, totally new lexical units are compared with multiword items that are made up of familiar forms. In the second experiment, different types of new senses of familiar forms are compared. Both studies underscore the importance of knowledge of form—but not of previously learned meaning—for the learning of new meanings for familiar forms.