Purpose: To extend formal models of language learnability to applications in clinical treatment of children with functional phonological delays. Method: The focus of the narrative review is on phonological complexity. This follows from learnability theory, whereby complexity in the linguistic input to children has been shown to trigger language learning. Drawing from the literature, phonological complexity is defined from epistemic, ontological, and functional perspectives, with specific emphasis on the application of language universals in the selection of target sounds for treatment. Results: The cascading effects of phonological complexity on children's generalization learning are illustrated, and frequently asked questions about complexity in treatment are addressed. Conclusion: The role of complexity in cognitive development is introduced to demonstrate the apparent robustness of effects. (Contains 2 tables and 3 figures.)