Metastasis is the most lethal attribute of cancer cells and clinical decisions regarding treatment are based largely upon the likelihood of developing metastases. However, improvements in detection as well as recent experimental data have raised questions about the most appropriate definition of a metastasis, especially whether the mere presence of cells at secondary sites constitute a metastatic lesion. After reviewing the experimental basis of metastasis, a definition of metastasis is proffered along with a proposal to consider regarding modification of staging parameters. © 2006/2007 IOS Press. All rights reserved.