The objective of this paper is to examine the concepts of holism and reductionism as they relate to System Dynamics (SD) and to a lesser extent Systems Thinking (ST) then to relate the findings of that examination to some of the disillusionment with SD and the resultant lack of commercial take up. This paper looks at the concepts of Reductionism and Holism as applied throughout the literature in a broad range of academic areas from medicine and philosophy to physics and looks at different definitions of holism, and reductionism. Practical applications of Reductionist and Holistic principles are examined. From this analysis the fundamental nature of SD as a Holistic/Reductionist methodology is deduced and differentiated from the more generally holistic nature of ST. From this basis the conclusion that SD is a reductionist methodology in practice, though not necessarily described as such by practitioners, is postulated. The paper recommends that there be further work carried out in the field of theoretical and practical holism and greater awareness of these issues within the practitioner community. Within the conclusions there is a pointer for some further fundamental areas of work.