Tumor progression is dependent on a number of sequential steps, including initial tumor-vascular interactions and recruitment of blood vessels (i.e., the angiogenic switch), as well as tumor cells interacting with the surrounding microenvironment and its different components. Failure of a microscopic tumor to complete one or more of these early stages may lead to delayed clinical manifestation of the cancer and a state of stable non-progressing disease (i.e., tumor dormancy). In this review, some of the clinical and experimental evidence is summarized, suggesting that microscopic human cancers, either primary, recurrent or metastatic, can remain in an asymptomatic, non-detectable, and occult state for a long period of time. We also review current experimental human tumor dormancy models which closely recapitulate clinically observed delay in tumor progress.