This study begins with a discussion of what a phonological rule is, and how the term has been used in previous investigations. The relation between the child's phonological system and the adult's system, the relationship between discrimination and production, and the viability of positing underlying forms for children's words are also discussed. Specific phonological rules are analyzed separately, with data presented from children learning English, French, and Czech. The study concludes with sample derivations of children's forms, showing the operation of phonological rules in describing the discrepancy between children's forms and the adult model. (Author)