This study documents the emerging ability of children to use demonstrative expressions and definite and indefinite articles in order to establish a clear reference for the hearer in a natural discourse situation. The analysis is based primarily on transcripts of twins recorded over a period of nine months when they were 33 to 42 months old. Additional data are drawn from conversations of three other children interacting with siblings aged 3 years to 5 years 4 months. Each transcript was coded and analyzed for errors and omissions. It was found that children rarely made errors in using the determiners; the errors occurred in omitting them. The results of the analyses show a clear, four-stage progression in the acquisition of determiners. It appears that the stages in the acquisition of definiteness and indefiniteness are marked by three characteristics: (1) indefinite articles are used throughout the data; (2) expressions which would require demonstrative determiners and definite articles in adult language are absent in the first stage of child language; and (3) several potential uses of the definite articles are unexploited in the children's language. A question remaining to be considered is that concerning strategies inherent in article usage. (AMH)