It is widely agreed that much second language vocabulary learning occurs incidentally while the learner is engaged in extensive reading. After a decade of intensive research, however, the incidental learning of vocabulary is still not fully understood, and many questions remain unsettled. Key unresolved issues include the actual mechanism of incidental acquisition, the type and size of vocabulary needed for accurate guessing, the degree of exposure to a word needed for successful acquisition, the efficacy of different word-guessing strategies, the value of teaching explicit guessing strategies, the influence of different kinds of reading texts, the effects of input modification, and, more generally, the problems with incidental learning. This article briefly surveys the empirical research that has been done on these issues in recent years.