The principle of equivalence of gravity and acceleration, or gravitational and inertial mass is the fundamental basis of general relativity. This was Einstein's key insight. Professor Susskind begins the first lecture of the course with Einstein's derivation of this equivalence principle. He then moves on to the mathematics of general relativity, including generalized coordinate transformations and tensor analysis. This topic includes the important point that the determination as to whether a spatial geometry is flat (i.e. Euclidean) is equivalent in some respects to the determination of whether an object is in a gravitational field, or merely an accelerated reference frame.