Fluorescence Correlation Spectroscopy (FCS) is a fluorescent spectroscopic technique that can be applied to measure the flow dynamics of a microfluidic channel. The principle of FCS is fluorescent molecules traveling through a tightly focused light volume inside a fluidic channel causes the fluorescent signal to fluctuate as the fluorescent molecules enter and exit the laser light focus. The flow parameter of the fluidic channel can be extracted from the autocorrelation function calculated from the measured fluorescent fluctuation. Several geometrical variations of FCS have been implemented to non-invasively map the flow characteristics of microfluidic devices in various ways to aid future designs of fluidic devices. Combining cross-correlation FCS (uses two types of fluorescent molecules) with microfluidic devices allows several important biomedical applications, including antigen detections and DNA-protein binding studies. In this article, the theory of FCS and its biomedical applications are reviewed.