In Part I of this article, the complex relationship between chronic pain and depression is studied from different theoretical points of view. First, some problems concerning the prevalence and the diagnosis of the co-existent depression are discussed. Then the `two-way' relationship: `chronic pain depression', and `depression chronic pain' is more closely examined. Since the latter aspect is very controversial and rather neglected in the literature, it will be studied here in depth. After this, some recent hypotheses concerning a possible common biological basis of chronic pain and depression are mentioned. Finally some concluding remarks are formulated concerning the nature and the `direction' of the relationship between chronic pain and depression, which may be considered as paradigmatic for a unifying theory of chronic pain.