Four studies investigated adults' uses and children's acquisition of the two Spanish synonyms of to be, ser and estar. The first study consisted of a distributional analysis of ser and estar in Spanish spoken by children and adults. The results revealed that the copulas were used contrastively across different syntactic contexts. Forms of ser were used exclusively with nominals, forms of estar were used as auxiliaries and with locations, and both forms were used with adjectives. Study 2 documented a semantic difference between ser and estar with adjectives for adults such that adjectives that were labelled with ser were weighed more heavily than attributes that were labelled with estar in a categorization task. In Study 3, a semantic contrast between objects and events with locations in adults was empirically demonstrated. Study 4 examined the use of the Spanish copulas with adjectives and locations by Spanish-speaking children from 3 to 11 years of age. The findings from Study 4 suggest that Spanish-speaking children honored fewer semantic contrasts than adults, and that their uses have a more syntactic basis. The study's implications for the structure of language and its acquisition are discussed.