Microdevices are disclosed to efficiently, accurately, and rapidly isolate and enumerate rare cells, such as circulating tumor cells, from liquids such as whole blood. The system employs multiple parallel meandering channels having a width on the order of 1-2 cell diameters. The microdevices can be produced at low-cost, may readily be automated, and in many instances may be used without pre-processing of the sample. They may be used to isolate and enumerate rare cells, including for example the detection and diagnosis of cancers, cancer staging, or evaluating the effectiveness of a therapeutic intervention, or detecting pathogenic bacteria. The device may optionally be used to nondestructively capture and later to release target cells.