The natural phonology theory, related to European structuralism, makes two fundamental assumptions: (1) phonemes are mental images of the sounds of language, and (2) phonological processes represent subconscious mental substitutions of one sound or class of sounds for another that are the natural response to the relative difficulties of sound production. The processes are either fortitions, that is, they ensure perceptual clarity in pronouncing different words; or they are lenitions, that is, they represent change toward articulatory simplicity enabling the vocal apparatus to do less work. It is posited that these processes are not learned by speakers in acquiring their language, but are a universal response to difficulties presented by physical and perceptual limitations of human nature. Evidence is reviewed showing that these processes are found in the description of the development of various languages, and in the pidginization process. Difficulties encountered in first and second language learning seem to bear out the natural phonology theory. Natural phonology predicts that second language learners will substitute "easier" sounds for those that do not exist in their native languages, that they will treat similar sounds as if they were the same as those in their native language, and that some errors cannot be attributed to interference because they are due to the operation of universal phonological processes. (AMH)