Previous vocabulary research has focused on the size of lexicons and the number of words learned through various activities. To date, this type of research has generated little understanding of how individual words are acquired. To explore this issue, I tracked the acquisition of 11 words over the course of a year for 3 adult learners with advanced proficiency in English. I measured 4 kinds of word knowledge: spelling, associations, grammatical information, and meaning. The participants had little problem with spelling, but rarely knew all of a target word's meaning senses or derivational word forms. Knowledge of the meaning senses of the target words improved about 2.5 times more than it was forgotten, and some of the word knowledge types appear to be interrelated. However, the study did not show evidence of a developmental hierarchy for word knowledge types.