This chapter reviews findings from the field of first language acquisition that shed light on morphological theory. The focus is on the acquisition of multimorphemic words and the distinction between regular and irregular inflection. The first aim of the chapter is to show that child data should always be interpreted against the background of developments in other areas that may influence children’s morphological performance. Child data have played a role in the debate about single versus dual route processing, which overlaps in part with the theoretical distinction between full and impoverished lexical entries. The second aim of the chapter is to provide a balanced overview of the empirical arguments that may support these different theoretical views. Specific attention is paid to the interpretation of developmental errors, languages other than English, and patterns observed in atypical language learners.