This theme involves a context of discovery, from 1905-1915, as well as a context of justification that unfolded afterward. In 1907, when he first began to contemplate a theory of gravitation and inertia based on a generalized theory of relativity, Einstein was guided by two physical ideas: his equivalence principle and the notion that inertial effects were related to fields generated by moving bodies. He later added a third, mathematical demand: the principle of general covariance. The latter was inspired by the Ricci calculus, which Einstein first learned about from his friend Marcel Grossmann in 1912. He quickly attached deep physical meaning to this principle, seeing it as a royal road to a general theory of relativity. When he made his momentous breakthrough to a generally covariant theory of gravitation in November 1915, however, the foundations of general relativity were by no means secure. After tracing Einstein's journey up through November 1915, we turn to the context of justification in the early history of GRT. During the period 1916-1920 several key contributions to GRT were made by leading mathematicians and astronomers, including Karl Schwarzschild, David Hilbert, Willem de Sitter, Hermann Weyl, and Felix Klein. However, the physical, mathematical, and conceptual foundations of GRT continued to arouse passionate debates, few of which helped to clarify any fundamental issues.