Hilbert's 1915 unified theory of matter and gravitation was established on two main pillars: (1) the electromagnetic theory of matter developed by Gustav Mie in 1912–13, and (2) Einstein's recent attempts to generalize the principle of relativity and to include gravitational phenomena within it. The present chapter provides an overview of Einstein's relevant activities during this time (§ 6.1), and, somewhat in greater detail, Mie's theory (§ 6.2). This theory, as will be seen (§ 6.4), did not actually attract Hilbert's attention in its original form, but rather only after Max Born reformulated it and presented it in Göttingen. The discussion of the general, relevant context is complemented in this chapter with a brief overview of contemporary debates on gravitation (§ 6.3). These four sections together provide the necessary background for understanding the events of June-November 1915 in Göttingen, when Hilbert presented the first installment of his unified theory, and what followed thereafter.