Abstract: Null subject phenomena have a special place in syntax and in language acquisition. (‘Null’ subjects are the absence of an overt subject before a verb that is tensed.) The null subject parameter was one of the first to be suggested within the principles-and-parameters framework and was the first to be investigated in child language. As of this writing, we know that there is not just one null subject parameter, nor just two. We know that there is extensive cross-linguistic variation in when subjects do and do not appear, much more variation than the early comparison of English and Italian suggested. The variation is so extensive that there is reason to question whether one should speak of parameters at all in connection with null subjects. Nevertheless, linguistic theory has to account for that variation. The sentential relation ‘subject of’ is a fundamental property of sentences. Just as hypotheses about the syntax of null subjects have proliferated, so have hypotheses about children’s acquisition, and so have hypotheses about the relation between syntactic theory and acquisition theory. This chapter focuses on what we know so far about the cross-linguistic acquisition of subjects and discusses the hypotheses that have been developed to account for the facts of acquisition. It briefly reviews hypotheses about the syntax of null subjects and discusses the relation between syntactic theory and language acquisition.