Incidental vocabulary acquisition refers to the learning of vocabulary as a by-product of any activities not explicitly geared to lexical learning. This study summarizes and reviews the studies of incidental L2 vocabulary acquisition in the past three decades from the following aspects: its source, linguistic and nonlinguistic factors affecting incidental vocabulary acquisition, and Focus-on-Form tasks aiming to enhance incidental vocabulary acquisition. This article also makes predictions of future trends in this field from the perspectives of spoken discourse, multi-mode discourse and multi-lexical unit.