This Element aims to address the complexity of metalinguistic awareness to achieve a thorough account of its impacts on second language (L2) reading development and promote an in-depth understanding of the factors regulating the influence of first language (L1) metalinguistic awareness on L2 reading. It is guided by four questions: 1) To what extent do L1 phonological, orthographic, and morphological awareness correlate with L2 phonological, orthographic, and morphological awareness in L2 readers? 2) To what extent do phonological, orthographic, and morphological awareness correlate with word decoding intralingually in L2 readers? 3) To what extent do L1 phonological, orthographic, and morphological awareness correlate with L2 word decoding in L2 readers? 4) To what extent do the relations in questions 1–3 vary as a function of linguistic-, learner-, measurement-, and instruction-related factors? This Element is the first to systematically investigate the roles of distinct facets of metalinguistic awareness in L2 reading.