Metastasis, the spread and growth of tumours at secondary sites, is an extremely important clinical event, since a majority of cancer mortality is associated with the metastatic tumours, rather than the primary tumour. In spite of the importance of metastasis in the clinical setting, the actual process is extremely inefficient. Millions of tumour cells can be shed into the vasculature daily; yet, few secondary tumours are formed. The classical hypothesis explaining the inefficiency was a series of secondary events occurring in the tumour, resulting in a small subpopulation of cells capable of completing all of the steps required to successfully colonise a distant site. However, recent discoveries demonstrating the ability to predict metastatic propensity from gene expression profiles in bulk tumour tissue are not consistent with only a small subpopulation of cells in the primary tumour acquiring metastatic ability, suggesting that metastatic ability might be pre-programmed in tumours by the initiating oncogenic mutations. Data supporting both of these seemingly incompatible theories exist. Therefore, to reconcile the observed results, additional variables need to be added to the model of metastatic inefficiency. One possible variable that might explain the discrepancies is genetic background effects. Studies have demonstrated that the genetic background on which a tumour arises on can have significant affects on the ability of the tumour to metastasise and on gene expression profiles. Thus, the observations could be reconciled by combining the theories, with genetic background influencing both metastatic efficiency and predictive gene expression profiles, upon which, subsequently, metastasis-promoting mutational and epigenetic events occur. If the genetic background is an important determinant of metastatic efficiency, it would have significant implications for the clinical prediction and treatment of metastatic disease, as well as for the design of potential prevention strategies.