A review is presented of studies of fatigue in fibres and plastics. The nature of fatigue in visco-elastic systems is discussed, and the reasons for the use of cumulative extension testing are given. A model is described showing the importance of imperfect elastic recovery and of time-dependent effects in determining behaviour in a fatigue test. Superimposed on this, there may be some true fatigue effects. Using this model, a more rational explanation of experimental results, which are otherwise confusing, is attempted.