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.