This thesis examined variations in the word decoding skills of children with reading disabilities. These variations were related to possible cognitive, developmental, and environmental causes of reading disability. Possible implications for educational interventions were also analyzed in the five studies. The thesis critically examines the inclusion of the concept of intelligence in the definition of developmental dyslexia. It is suggested that variations in word decoding skills should offer a more solid basis for a study of varieties of reading disability. The empirical studies showed that: (1) in young children there was a shift from phonological to orthographic word decoding; (2) phonological type children (weak in phonological decoding) were characterized by specific phonological deficits; (3) surface type children (weak in orthographic decoding) showed more global cognitive deficits suggesting a general developmental delay; (4) surface type children showed impaired visual implicit memory for words, which might be associated with limited print exposure; (5) an improvement in phonological awareness only transferred to an improved text reading ability for some children with reading disabilities; and (6) children who did not benefit from a phonological intervention seemed to rely on orthographic word decoding in text reading. (Each of the five studies contain references.) (CR)