This study addresses the issue of underspecificity in undergraduate writing. Two corpora of South African undergraduate essays (writing in first and second language English) are compared to a corpus of academic papers (PW). The comparison is in terms of corpus-analytic methods and a discourse analytic approach to using definite expressions. So-called "abstract" nouns like problem, purpose, fact, example, and idea, identified as "carrier/shell" nouns, are often claimed to cause 'vagueness' in student writing, so students are advised not to use them. Yet corpus analysis (e.g. Biber et a!, 1999) shows they are a core feature of English academic vocabulary. Distinguishing between sense and reference clarifies a crucial difference between nouns with vague denotation, and noun phrases (NPs) with vague reference. A quantitative analysis of the corpora reveals that the students and PWs use both "shell" nouns, and the syntactic patterns in which they frequently occur, with similarly high frequency. However, the PW sub-corpus contains significantly more nouns than the student corpora and exhibits significantly more variety in the nouns used. Using a discourse-based approach, a second analysis focuses more closely on the discoursal structure of the texts, concentrating on definite referring expressions. A specially developed method of coding is used to categorise the way writers specify and constrain the referents of NPs containing 'abstract' nouns. Student writers in this study tend to use "shell" nouns not only as NP heads but, repetitively, within specifying phrases whose function should be to constrain reference. Hawkins' (1991) approach to definiteness is used to show that this embedding of 'vague' nouns in referring NPs, together with other discoursal features, stems from student writers inappropriately assuming shared knowledge. This in turn strengthens the argument that if student writers are to be viewed as apprentice PWs, learning to write for a 'general readership' must be seen as a crucial part of their training.