This Element discusses the complexity of metalinguistic awareness (i.e., phonological, orthographic, and morphological) and provides a thorough account of its impact on second language (L2) reading development. In particular, it underscores various factors regulating the influence of first language (L1) metalinguistic awareness on L2 reading. It presents the most comprehensive meta-analysis of inter-lingual correlation coefficients between L1 and L2 metalinguistic awareness on the one hand and those between L1 metalinguistic awareness and L2 reading on the other. This Element is the first to systematically investigate the roles of distinct facets of metalinguistic awareness in L2 reading.