Although the diagnostic testing of writing is still quite primitive, error analysis and protocol analysis hold promise for writing diagnosis. True diagnostic testing does more than aid in placing students; it identifies the nature of needed instruction. Tests in writing have not been developed to reflect approaches used in diagnostic testing in other areas. Reviews of existing standardized writing tests suggest shortcomings in all three categories of tests identified: objective tests, objective tests corresponding to individualized writing sequences, and essay tests. Information about error analysis, one of the areas that holds promise for writing diagnosis, has been drawn from teachers of English as a second language, who have moved from corrective error analysis to contrastive analysis and finally to the analysis of errors as clues to inner processes. The other promising area, protocol analysis, involves analyzing subjects' oral descriptions of everything they think while performing a task, as a means of identifying inner processes and obstacles encountered. An examination of the writing protocol of a subject in an experimental study reveals the process through which a writer understands a topic. Although protocols are incomplete representations of inner processes, they provide far more information about writing processes than does simply examining the writing outcome. (The paper includes 27 lines of the writing protocol discussed and a chart showing various approaches to error analysis.) (GT)