Two models of the gravitational and inertial interactions are considered in this paper, the Newtonian model and the TI field model. We’ll see how each model addresses the equivalence principle and the three properties of mass: passive gravitational mass, active gravitational mass and inertial mass. In the Newtonian model the ratio of passive gravitational mass to inertial mass of an object falling at relativistic speed toward a black hole changes, so the often stated equivalence of these two properties of mass is not upheld. In the Newtonian model, absent extraneous forces, all objects moving at non- relativistic speed in a gravitational field accelerate at the same rate because the ratio Mp / MI of the passive gravitational mass to the inertial mass is the same for all objects regardless of their intrinsic mass or constitution. The ratio Mp / MI of the passive gravitational mass to inertial mass need not be unity for the acceleration of objects in a gravitational field to be the same for all objects regardless of size or constitution. In the common expression for the acceleration profile about a gravitational body the ratio Mp / MI of the passive gravitational mass to the inertial mass is sequestered within the gravitational constant G itself. The TI field model of gravity and inertia unifies the nature of the gravitational and inertial interactions through the acceleration of massive particles relative to the TI field, an acceleration that is common both to the acceleration of an object in response to an external force and the acceleration of the TI field in response to gravity. The TI field model unifies the nature of the gravitational and inertial interactions not by equating the numerical values of passive gravitational mass and inertial mass, but by the relative acceleration of the TI field and matter particles that underlies both