(a) Microscopy IVFC design, reprinted with permission from (Novak et al., 2004; Georgakoudi et al., 2004). A laser is focused across a blood vessel in the ear of a mouse. (b) As fluorescently-labeled cells pass through the slit, transient fluorescence pulses are generated and detected. Example applications of IVFC, including, (c) the effect of treatment on homing of leukemic cells, figure reprinted with permission from (Sipkins et al., 2005), (d) the number of circulating MM cells, figure reprinted with permission from (Runnels et al., 2011), and, (e) circulating lung tumor cells during disease progression and response to treatment, figure reprinted with permission from (He et al., 2007).