In the simplest sense, reading acquisition is a process of learning how a writing system represents a spoken language. It requires conscious awareness of how the basic graphic symbol, the grapheme, represents a spoken unit. However, unlike language acquisition, which occurs naturally, rapidly, and automatically in most children, learning to read is a lifetime developmental process that requires effort, explicit instruction, and conscious awareness of the language structure (known as metalinguistic awareness). Many children fail to achieve full proficiency in reading, and some exhibit specific problems with reading, writing, and spelling despite adequate intelligence, motivation, and educational opportunity. This specific learning difficulty, known as developmental dyslexia, has varying incidence rates depending on the diagnostic criteria and the language examined. The divergent incidence rates, together with some findings suggesting that dyslexia has different behavioural and neural manifestations within and across cultures, has led to the question of whether the underlying causes of dyslexia differ across writing systems and whether it is possible to be dyslexic in one language and not dyslexic in another.