It is widely agreed that much second language vocabulary learning occurs incidentally whilethe learner is engaged in extensive reading. After a decade of intensive research, however, theincidental learning of vocabulary is still not fully understood, and many questions remainunsettled. Key unresolved issues include the actual mechanism of incidental acquisition, the typeand size of vocabulary needed for accurate guessing, the degree of exposure to a word needed forsuccessful acquisition, the efficacy of different word-guessing strategies, the value of teachingexplicit guessing strategies, the influence of different kinds of reading texts, the effects of inputmodification, and, more generally, the problems with incidental learning. This article brieflysurveys the empirical research that has been done on these issues in recent years.