In the previous chapter we looked at how we use and understand language, now let’s look at the ways that people lose language and also experience language difficulties of various kinds. These aren’t the normal errors and mistakes we’ve already discussed, but are the result of disability, disease, disorder, brain injury, and other factors that can affect speech use and understanding and further impair the ability to read and write. In the US alone, between five to ten percent of the population have various types of communication disorders. The figures are even higher for learning disorders such as dyslexia. All in all, this amounts to millions of people who live with language differences. This chapter discusses how both developmental and acquired disorders affect language processing and production.