We provide an exhaustive review of studies in the relatively new domain of research on the influence of orthography on second language (L2) phonological acquisition. While language teachers have long recognized the importance of written input--in addition to spoken input--on learners' development, until this century there was very little systematic research investigating the relationship between orthography and L2 phonological acquisition. Here, we review studies of the influence of written input on L2 phonological awareness, phoneme perception, the acquisition of phonological processes and syllable structure, and the pronunciation and recognition of words. We elaborate the variables that appear to moderate written input effects: (1) whether or not a novel phonological contrast is systematically represented by the L2 writing system (systematicity); (2) whether some or all of the L2 graphemes are familiar to learners from the L1 (familiarity); (3) for familiar graphemes, whether the native language (L1) and the L2 employ the same grapheme-phoneme correspondences (congruence); and (4) the ability of learners to perceive an auditory contrast that is systematically represented in writing (perceptibility). We conclude by calling for future research on the pedagogical implications of this body of work, which has thus far received very little attention by researchers.