Early dissemination of cancer cells from the primary tumor via the circulatory system may result in the formation of microscopic metastatic deposits (micrometastases, MMs) in secondary compartments such as the bone marrow (BM), where there is a favorable environment for their subsequent growth and spread. MMs are considered the main reason for metastatic relapse in patients with early stage solid cancers after resection of the primary tumor. Although the molecular pathways leading to MMs remain only partly understood, there is increasing evidence that the detection of MMs in BM aspirates at the time of primary diagnosis is an independent prognostic factor, with a major influence in the stratification of these patients for adjuvant clinical treatment. Further potential applications of the detection of MMs include their use in monitoring therapeutic response or even in revealing targets for novel systemic therapies. All these intriguing possibilities are intensely investigated and carry great promise for radical improvements in the assessment and treatment of several epithelial cancers which are currently to blame for the majority of cancer-related deaths in the industrialized world.