We discuss a range of metric theories of gravity and their post-Newtonian limits. We begin with a general recipe for calculating the post-Newtonian limit in generic metric theories, and then turn to specific theories. Included are general relativity, scalar-tensor theories, vector-tensor theories (including Einstein-Aether and Khronometric theories), tensor-vector-scalar theories (including TeVeS), quadratic gravity theories (including Chern-Simons theory), and massive gravity theories. We also review the fate of theories of gravity that were featured in the first edition of this book, but that are no longer considered viable or interesting, including Whitehead’s theory and Rosen’s bimetric theory.

We discuss the foundations of general relativity and all modern gravitational theories, based on the Einstein Equivalence Principle (EEP). We show that this principle is the basis for all metric theories of gravity including general relativity, in which gravity is a consequence of spacetime geometry. We review experimental test of the three pieces of EEP, the Weak Equivalence Principle, Local Lorentz Invariance and Local Position Invariance, and describe a number of general theoretical frameworks used to analyse EEP and its consequences.