The process of metastasis consists of a series of linked, sequential steps that must be completed by tumour cells if a metastasis is to develop. Although some of the steps in this process contain stochastic elements, metastasis as a whole favors the survival and growth of a few subpopulations of tumour cells that pre-exist within the heterogeneous parent neoplasm. Metastases can have a clonal origin, and different metastases can originate from the proliferation of single cells. The outcome of metastasis depends on the interaction of metastatic cells with different organ environments. Organ-specific metastases have been demonstrated in a variety of experimental tumour systems, and tumour growth can be specific to a particular site within one organ. The factors that regulate metastasis are the intrinsic properties of some tumour cells and host factors involved in homeostasis. Understanding these factors should allow for the development of more effective of cancer metastasis.