The chapter looks at definitions of dyslexia and argues that definitions confined to reading deficits are limited for understanding the complex range of difficulties faced by the adult student at university. As such, definitions provided by the Rose Report (2009), and the British Dyslexia Association (2007) describing core characteristics of dyslexia and co-occurring difficulties are presented as more appropriate for comprehending the spectrum nature of dyslexia. The chapter also draws attention to Elliott and Grigorenko’s (2014) claim that dyslexia does not exist. This is dismissed by arguing that their concept of dyslexia is perceived as a reading disability in school-aged children and is not applicable to understanding adult dyslexia. The refutation of Elliott and Grigorenko is reinforced by outlining evidence of the existence of dyslexia from empirical work on causal theories. It is suggested, however, that causal theories looking for a single deficit are less suitable for characterising adult dyslexia than models such as Pennington’s multiple deficit model. Finally, a picture is provided of adult dyslexia by considering ways various causal theories manifest in different subgroups of students with dyslexia. Types of academic areas affected by these various deficits are also specified.