Variability is a term used to cover several types of phenomena in language sound patterns and in phonetic realization of those patterns. Variability refers to the fact that every repetition of an utterance is different, in amplitude, rate of delivery, formant frequencies, fundamental frequency or minor phase relationship changes across the sound spectrum. Articulator movements, muscle contractions and neural signals also vary. The theory of the phoneme was developed to explain the phenomenon of variation, although problems arose in defining the range to be covered and in classifying variations. The sciences of phonetics and phonology diverged, with phonetics dwelling more on factual accuracy and phonology on theory. The two branches are now more compatible, and must be for their data to match. The emphasis in phonetics has switched from how we speak to what we need to know to speak. Empirical evidence suggests that some constraints and speech variations are intentional. It is clear that major constraints on phonology are provided by the neural processing limitations of the brain and by knowledge of the properties of the speaking apparatus. (CHK)