Income mobility may be seen as arising from two sources: (i) the transfer of income among individuals with total income held constant, and (ii) a change in the total amount of income available. In this paper, we propose several sensible properties defining the concept of income mobility and show that an easily applicable measure of mobility is uniquely implied by these properties. We also show that the resulting measure is additively decomposable into the two sources listed above, namely, mobility due to the transfer of income within a given structure and mobility due to economic growth or contraction. Finally, these results are compared and contrasted with other mobility concepts and measures in the literature.