Relative clauses have been implicated alternately as a strength and a weakness in the language of people with Williams Syndrome (WS). To clarify the facts, an elicited production test was administered to 10 people with WS (age 10-16 years), 10 typically developing children (age 4-7 years), and 12 typically developing adults. Nearly every WS participant successfully produced both subject gap (SG) and object gap (OG) relative clauses, and produced relative clauses in both right-branching and centre-embedded positions. However, OG relative clauses were produced less frequently than SG relative clauses, and trials targeting OG relative clauses often elicited contextually inappropriate responses. Critically, these two patterns were also observed in typically developing children, though at attenuated levels. It is argued that by late childhood people with WS know the structure and the meaning of relative clauses, but qualitatively normal processing factors cause unusually severe interference in some subprocesses of sentence production. (Contains 10 tables and 5 figures.)