Identical and fraternal twin pairs reared together have been key to understanding the genetic and environmental etiology of dyslexia and of individual differences in reading. In this chapter, we begin with a brief overview of the methods of twin research, and the historical development and application of these methods to understanding the etiology of individual differences and deficits in reading and related skills. Then we examine results from predominantly English-language twin research on dyslexia. The next section on twin studies of individual differences in reading ability introduces a broader cross-language perspective that includes comparisons of findings from studies in the United States, the United Kingdom, Australia, Norway, Sweden, the Netherlands, and China. Then we expand the reading phenotype beyond word recognition to reading comprehension, the ultimate goal of reading.