Metastasis is the leading cause of cancer-related death, involving a complex process where cancer cells must overcome multiple barriers to colonize distant organs. This process is highly inefficient, with only a small fraction of circulating cancer cells surviving to establish metastases. Despite this inefficiency, once metastases are established, current treatments often fail to provide durable responses. Understanding the molecular mechanisms underlying metastatic colonization is crucial for developing better prevention and treatment strategies. Key steps in metastatic colonization include cancer cell invasion, migration, entry into the circulation, and survival in distant organs. Recent advances in experimental models, imaging technologies, and genomic sequencing have provided new insights into the molecular mechanisms that allow cancer cells to invade, settle, and eventually overtake host tissues. These advances have improved our understanding of the metastatic process and informed the development of more effective treatments. The inefficiency of metastasis is evident in clinical observations, such as the low number of overt metastatic lesions despite high levels of circulating tumor cells. Experimental models confirm that many cancer cells die after entering distant organs, and only a minority survive to form macro-metastases. Factors influencing the survival and tumour-initiating activity of disseminated tumour cells are key determinants of metastasis. Early steps in metastasis involve cancer cell invasion, migration, and entry into the circulation. Cancer cells may leave tumors as single cells or as cell clusters, and evidence suggests that distinct cancer cell clones can cooperate to promote mutual survival and metastatic ability. In the bloodstream, cancer cells face significant challenges, including shear forces, innate immunity, and oxidative stress. They can survive by associating with platelets and undergoing metabolic changes to withstand oxidative stress. Mechanical entrapment of cancer cells in capillaries is a key mechanism for cancer cell arrest before exiting into tissue. The composition of vascular walls influences where cancer cells extravasate, with liver and bone marrow capillaries (sinusoids) facilitating extravasation due to their fenestrated endothelial cells and discontinuous basal lamina. In contrast, lung and brain capillaries have tighter junctions and additional structural barriers. Tissue defenses against infiltrating cancer cells include immune surveillance by CD8 T-cells and natural killer (NK) cells. The composition of the organ's immune cell population can influence its susceptibility to metastasis. For example, the liver is rich in NK cells, and their depletion increases hepatic metastasis. Supportive niches are crucial for the survival and tumour-initiating potential of metastatic cells. These niches provide signals that balance stem cell proliferation and quiescence, and are rich in developmental and self-renewal signals. Tumour cells can exploit these niches to survive and proliferate in distant organs. Pre-metastatic niches, created by systemic signals from the primary tumour, can support the immediate outgrowth of disseminated cancer cells. These niches are influenced by factors such as tumour-derived inflammatoryMetastasis is the leading cause of cancer-related death, involving a complex process where cancer cells must overcome multiple barriers to colonize distant organs. This process is highly inefficient, with only a small fraction of circulating cancer cells surviving to establish metastases. Despite this inefficiency, once metastases are established, current treatments often fail to provide durable responses. Understanding the molecular mechanisms underlying metastatic colonization is crucial for developing better prevention and treatment strategies. Key steps in metastatic colonization include cancer cell invasion, migration, entry into the circulation, and survival in distant organs. Recent advances in experimental models, imaging technologies, and genomic sequencing have provided new insights into the molecular mechanisms that allow cancer cells to invade, settle, and eventually overtake host tissues. These advances have improved our understanding of the metastatic process and informed the development of more effective treatments. The inefficiency of metastasis is evident in clinical observations, such as the low number of overt metastatic lesions despite high levels of circulating tumor cells. Experimental models confirm that many cancer cells die after entering distant organs, and only a minority survive to form macro-metastases. Factors influencing the survival and tumour-initiating activity of disseminated tumour cells are key determinants of metastasis. Early steps in metastasis involve cancer cell invasion, migration, and entry into the circulation. Cancer cells may leave tumors as single cells or as cell clusters, and evidence suggests that distinct cancer cell clones can cooperate to promote mutual survival and metastatic ability. In the bloodstream, cancer cells face significant challenges, including shear forces, innate immunity, and oxidative stress. They can survive by associating with platelets and undergoing metabolic changes to withstand oxidative stress. Mechanical entrapment of cancer cells in capillaries is a key mechanism for cancer cell arrest before exiting into tissue. The composition of vascular walls influences where cancer cells extravasate, with liver and bone marrow capillaries (sinusoids) facilitating extravasation due to their fenestrated endothelial cells and discontinuous basal lamina. In contrast, lung and brain capillaries have tighter junctions and additional structural barriers. Tissue defenses against infiltrating cancer cells include immune surveillance by CD8 T-cells and natural killer (NK) cells. The composition of the organ's immune cell population can influence its susceptibility to metastasis. For example, the liver is rich in NK cells, and their depletion increases hepatic metastasis. Supportive niches are crucial for the survival and tumour-initiating potential of metastatic cells. These niches provide signals that balance stem cell proliferation and quiescence, and are rich in developmental and self-renewal signals. Tumour cells can exploit these niches to survive and proliferate in distant organs. Pre-metastatic niches, created by systemic signals from the primary tumour, can support the immediate outgrowth of disseminated cancer cells. These niches are influenced by factors such as tumour-derived inflammatory