There is not one single system dynamics approach. Apart from 'mainstream' system dynamics, there are several distinctive practices in use. An attempt is made here to reveal basic assumptions of different practices and to classify them in a paradigmatic classification framework on the basis of these basic assumptions. Revealing its basic assumptions might be important for increasing the acceptance and use of system dynamics. It will be shown here that mainstream system dynamics corresponds well with critical pluralism, that other approaches are more postpositivist, pragmatist or constructivist, and that still other (non-existent) paradigmatic approaches might potentially be interesting. The classification framework might be used furhtermore to find (i) approaches that suit issues, circumstances, parties involved and goals, (ii) the basic assumptions of different approaches and hence the interpretation and use of the results, and (iii) consistent matches and mixes of system dynamics approaches with other method(ologie)s.