A taxonomy of revision changes was developed and applied to 18 case studies of writers' revisions. Subjects were six inexperienced student-writers, six advanced student-writers, and six expert adult-writers. The primary distinction of the taxonomy was between surface (formal and meaning-preserving) revisions and text-base (microstructure and macrostructure) revisions. Over a three-day span, the subjects (1) thought and made notes about the topic, (2) wrote an essay, and (3) wrote a revision/second draft. In addition, the expert writers made revisions of the inexperienced writers' first drafts. The results showed certain characteristic differences in how the different groups of writers revised their work. The experts turned out to be the most infrequent revisers. The inexperienced writers' changes were overwhelmingly surafce changes, while the experts' changes were more evenly distributed between surface and text-base changes. The advanced students were the most frequent revisers of the three groups, making surface changes as often as did the inexperienced writers and text-base changes much like those of the experts. The experts' changes of the inexperienced writers' first drafts were predominantly macrostructure changes, particularly additions, substitutions, and distributions. The experts were much more uniform in these revisions than in their revisions of their own texts. (RL)