I offer an investigation of the historical, methodological and epistemological aspects of the higher dimensional unification program in physics (HDUP). I examine the emergence and development of HDUP under two main periods: the classical period which concerns the five-dimensional Kaluza-Klein (KK) theory constructed by Theodor Kaluza and Oskar Klein in the twenties to unify gravity with electromagnetism; and the modern period which spans the late seventies to the early eighties and concerns the higher dimensional supergravity and superstring theories---aka modern KK theories ---aiming to unify gravity with the nuclear forces. My philosophical analysis of HDUP reveals different conceptions of unity as well as different mathematical structures underlying KK theories. I argue that the latter rest on a number of empirically unwarranted assumptions and a priori postulates. I use the conclusions of my investigation of HDUP to assess the merits of Philip Kitcher's unificationist model of scientific explanation. I construe Kitcher's account as requiring unification to lead to theories having comparatively greater explanatory and predictive power. In this sense, I take Kitcher's model of explanatory unification to be normative about what successful unification should consist of. I argue that both the original five-dimensional KK theory and modern KK theories---in their current state of the art---fail to meet Kitcher's normative standard of unification. Moreover, I argue that the relevant physics community also faults KK unification by using similar normative standards to Kitcher's for a successful unification. I conclude that Kitcher's account successfully captures the normative aspect of the practice of unification in the context of HDUP.
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