Selective attention (SA) is an important strategy for language comprehension both in L1 and L2. Much of the existing research on L2 listening and reading has identified SA as a distinctive ability of effective and skilled L2 listeners and readers. However, extensive and intensive research on this specific strategy is relatively sparse. Therefore, this article provides an in-depth review of previous research involving the SA mechanism in both L1 and L2 language comprehension. As a result, this article identifies that there are two-levels of SA existing in previous academic research: the word level and sentence level SA. They may represent different cognitive processes as well as different SA strategies in real practice. However, this kind of classification on SA has not been fully recognized and specifically pinpointed by previous literature, since previous studies focused on only one type of SA while ignoring the other. Therefore, this article proposes to classify these two kinds of SA and then reviews previous literature according to this novel classification. In addition, this article summarizes the most frequently investigated modulators of SA as well as commonly used research methods for SA-related research. Some possible research gaps are pinpointed accordingly. Finally, this article illustrates several concepts and models that could explain the SA mechanism from psychological and linguistic perspectives, which could serve as a theoretical framework in future study. This article may offer some inspirations for future academic research in this field and listening or reading practices in the real world.