Succession theory has played a central role in plant ecology since the beginning of this century. Community changes were interpreted with the classical succession paradigm. During the last ten years workers examined the empirical and experimental basis of the theory and found that it needed to be re-examined. This is the main theme of all sixteen contribution to this third volume in the series Advances in Vegetation Science, covering together with the second volume, the 1979 symposium of the Working Group for Data-Processing of the International Society for Vegetation Science. The first contribution, by Sjors, gives an introduction to the types of changes and criticizes right away one of the oldest concepts in succession theory, that of cyclical succession in bogs. Alternation of hummocks and hollows, which was for a long time considered to be the rule, must be regarded as the exception. Most structures are persistent.