Definitions that children provide can be a valuable measure of their syntax, and specifically, of their ability to produce relative clauses. This research explored the acquisition of subject, object, and indirect object relative clauses in 121 Hebrew-speaking children aged 3 years, 5 months to 8 years, 6 months (3;5-8;6). The children were asked to define 14 nouns, and their responses were collected and analyzed for various syntactic aspects. The main results were that children started using relative clauses in their definitions at age 3;8, and their use of relative clause increased consistently until they were 6 years old. Retesting 38 of the 6-year-olds at age 8;6 indicated no differences in several syntactic measures between their production of relative clauses at age 6 and 8;6, suggesting that the ability to produce relative clauses stabilizes around age 6. The participants made almost no grammatical errors at any of the ages, probably because they avoided the use of relative clauses when they had not mastered them yet. In the early stages participants produced mainly headless relatives, and with age the use of a relative head increased. The acquisition of relative clauses was not related to the ability to embed or to the ability to use pronouns: these abilities existed already in the youngest age group and remained constant throughout the age groups.