The disclosure relates to the enrichment and expansion of rare cells in blood such as circulating tumor cells (CTCs), cancer stem cells [CSCs] and other rare circulating cells. The microwells promote interactions between patient-derived CTCs and blood cells, allowing expansion of CTCs without the need for pre-enrichment or additional growth supplements. The cultured cells can be selected for propagation from single cells and have utility in drug screening, diagnostics and prognostics. The disclosure also includes a system comprising a cell enrichment device for enriching CTCs and CSCs and a device adapted to co-operate with the cell enrichment device to allow the testing of one or more agents, for example therapeutic or diagnostic agents.