This review summarizes evidence related to metastasis-associated genes and their functions. Their discovery, rigor of evidence for involvement in cancer metastasis, correlations and anticorrelations with metastatic propensity, biochemical function(s), and clinical associations are summarized and critically evaluated. What emerges is a picture whereby function is determined by context (tumor type, which in vivo or in vitro assays were used to assess function(s), whether RNA or protein is being measured, and redundancy of related molecules, cellular location, protein stability, etc.) due to differential regulation and interactions. Many metastasis genes have been implicated in individual steps of the metastatic cascade, while others impact multiple steps, complicating the ability to ascribe definitive mechanism(s) of action. Likewise, many metastasis-regulatory molecules are involved in multiple cellular signaling pathways due to interactions with other macromolecules. The implications of the complexity of metastasis genetics and epigenetics for prognosis and treatment are also briefly discussed.