The major problem for cancer patients is metastasis of the cancer from the primary tumor to secondary sites. Metastasis is the process by which tumor cells disseminate from the primary tumor, migrate through the basement membrane, survive in the circulatory system, invade into a secondary site, and start to proliferate. In the past, research had concentrated on the biology, taking more of a global view instead of a molecular view. More recently, the focus has been determining the molecular underpinnings, looking at genes that induce or inhibit metastasis. Metastasis suppressors, by definition, inhibit metastasis at any step of the metastatic cascade without blocking primary tumor growth. The expanding list of metastasis suppressors exist with every cellular compartment and have been shown to work by regulating signaling pathways that inhibit proliferation, cell migration and growth at the secondary site. Still, the biochemical basis of their inhibition is not completely known. Here we review the known metastasis suppressors and summarize the suspected mechanisms by which they inhibit metastasis.