Research in reading disabilities reveals that the most common source of reading difficulty is a core deficit in phonological processing. A deficit in phonological awareness impedes the acquisition of word recognition skills, which is the primary difficulty experienced by students with reading disability. Comprehension deficits in reading have received far less attention than word-level reading skills. Assessment of the various subskills of the reading process allows teachers to identify a student's pattern of reading difficulty and implement the appropriate reading intervention. Assessing the reading of non-words is the most effective method for determining a student's skill in phonemic decoding. When a student reads a nonsense word, he must rely on sound–symbol relationships and his knowledge of the alphabetic principle rather than on memorization. A comprehensive fluency assessment measures a student's reading rate, reading accuracy, and prosody. Letter knowledge assessments should include letter recognition tasks as well as the extent to which an individual knows the corresponding sound for a grapheme.