The aim of this study was to better understand the heterogeneity of developmental dyslexia by identifying the most common deficits in the reading systems of children with dyslexia with different poor word reading profiles. We classified the word reading profiles of 138 children with developmental dyslexia using nonword and irregular-word reading tests and then used independent experimental tests to explore the cognitive deficits within their word reading systems. The most common deficit associated with primary sublexical impairment (i.e., poor nonword reading) was poor grapheme-phoneme conversion (GPC) knowledge. The most common deficits associated with primary lexical impairment (i.e., poor irregular-word reading) were an impaired orthographic lexicon plus impaired links between this lexicon and the phonological lexicon and semantic knowledge. Finally, the most common deficits associated with mixed reading impairment (i.e., poor nonword reading and poor irregular-word reading) were poor GPC knowledge, an impaired orthographic lexicon, poor links between this lexicon and the phonological lexicon and semantic knowledge, and poor phonological output. We discuss the implications of these findings for theories of reading and for the diagnosis and treatment of dyslexia.