Intended to help reading instructors better understand the nature of reading errors, this report carefully examines one type of error--word omissions. Following a review of research in the area of omissions, the report discusses in detail the omission patterns, in context, of a single reader, then summarizes four key points about omissions demonstrated by that reader. Next, the report develops a major theoretical conclusion that omissions may be divided according to whether they are deliberate or nondeliberate. It compares the omission phenomenon to a related one, the substitution of nonwords for real words. The report then presents a categorization of the types and causes of nondeliberate omissions, followed by a discussion of the differential effect of omission on the reader's effectiveness. A discussion of how the understanding of omissions is helped by looking at the reader's retellings follows. The report concludes with a summary and the implications of the deliberate and nondeliberate theory of omissions for reading instruction. (HTH)