Poor polysyllabic word (PSW) production seems to mark paediatric speech impairment as well as impairment in language, literacy and phonological processing. As impairment in these domains may only manifest in PSWs, PSW production may provide unique information that is often excluded from clinical decision making because insufficient PSWs are included in speech tests. A 5-stage model of PSW acquisition is described. The model, grounded in optimality theory, expresses a reciprocal relationship between the relaxation of markedness constraints and the contraction of faithfulness constraints from 12 months of age to adolescence. The markedness constraints that persist to the age of 7;11 years are associated with non-final weak syllables and within-word consonant sequences. Output changes are argued to reflect increasing specification of phonological representations with age, liberating information for motor planning and execution, resulting in increasingly accurate output. The clinical implications of PSWs in assessment and therapy are discussed.