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The mass contained in an arbitrary spacetime in general relativity is notwell defined. However, for asymptotically flat spacetimes various definitionsof mass have been proposed. In this paper I consider eight masses and show thatsome of them correspond to the active gravitational mass while the otherscorrespond to the inertial mass. For example, the ADM mass corresponds to theinertial mass while the M$\o$ller mass corresponds to the active gravitationalmass. In general the inertial and active gravitational masses are not equal. Ifthe spacetime is vacuum at large $r$ the Einstein equations force the inertialand active gravitational masses to be the same. The Einstein equations alsoforce the masses to be the same if any matter that extends out to large $r$satisfies the weak, strong or dominant energy condition. I also examine thecontributions of the inertial and active gravitational masses to thegravitational redshift, the deflection of light, the Shapiro time delay, theprecession of perihelia and to the motion of test bodies in the spacetime.

On the Meaning of Various Mass Definitions for Asymptotically Flat Spacetimes