A discussion of young children's production of English utterances with missing constituents focuses on the omission of subjects. The theory that young children have different grammars from those of adults is disputed, and it is suggested that, instead, subjects are omitted due to performance factors. Processing limitations in child language are evidenced in early difficulties with utterance length, omission of other constituents, and some children's reduction of the subject to a schwa. A study of the speech of three children supported the processing theory's prediction that children's subjectless sentences would tend to have longer verb phrases than sentences with subjects. Therefore, in contrast to the notion that children acquiring English represent pro-drop grammars until they are 2 to 3 years old, it is proposed that children initially represent overt subjects as obligatory (non-pro-drop), and only when hearing subjectless sentences do they change their grammars to pro-drop, as in Italian. (MSE)