There are some phonological theories, but the two classic theories are generative phonology and natural phonology. Generative phonology includes the tenet that underlying phonological representations are translated into allophones according to the rules of grammar of the phonological system to create a surface phonetic representation. Natural phonology explains why children tend to simplify speech. Additionally, the enduring concept from Stampe’s theory of natural phonology is the phonological process. In current times, the term and types of phonological processes is simply used to describe error patterns in the speech of children. Contemporary theories of phonology include optimality theory, nonlinear phonology, and representation-based accounts of children’s speech. Each of these offers alternative explanations as well as descriptions of speech acquisition and SSD in children.