Vocabulary acquisition, after being neglected for centuries, aroused people's attention from the second half of last century. At that time, people began to realize, instead of grammar, vocabulary occupies the central role in language acquisition (Gass & Selinker, 1994). Compared with intentional vocabulary acquisition, incidental vocabulary acquisition was found to be the major way for people to acquire vocabularies. Early studies into incidental vocabulary acquisition focused on incidental vocabulary acquisition through reading activities. Later on, people found that listening activities was another good way to enhance incidental vocabulary acquisition. Nowadays, task mode of incidental vocabulary acquisition has become more pluralistic than before. This article is to review studies into incidental vocabulary acquisition through different input and point out the limitations of previous studies. The first limitation of previous studies is that word knowledge framework was undefined in previous studies and the second limitation is that prior knowledge, an factor which needs to be controlled, was neglected by some scholars. This review will hopefully provide some suggestions for both language teachers and language learners.