The use of biosensors has become a standard method to characterize biomolecular interactions. Data obtained from biosensor studies are widely used to evaluate drug candidates, particularly in relation to their binding properties towards a selected target. The importance of measuring such interactions in a biologically relevant environment has become the new challenge for the biosensor technologies. In this chapter we describe a method for studying interactions between proteins and targets at the surface of intact cells using a quartz crystal microbalance (QCM) biosensor.