The heterogeneous character of studies on this subject has already been mentioned and can be noticed even on an examination of the definitions and classifications of the
occurrences. Some experts divide pauses into individual and functional pauses, others describe them as silent and filled pauses. Another quite vague question concerns the relationship between pauses and hesitations. In order to cast light on these issues, studies on the subject will be
presented following the different approaches. It has to be stressed that in many of them non-fluencies are not the object of the study but only a useful tool to investigate psychological and cognitive mechanisms in communication and/or SI.