Optical coherence tomography (OCT) is an interferometric scanning technique. The instrument core is a Michelson interferometer comprising a moving reference mirror and a backscattering object representing the second mirror. The light source coherence length determines the depth resolution of this imaging method. OCT provides images with improved contrast and depth resolution compared to confocal microscopy in the case of turbid objects. Here, the imaging performance and contrast mechanisms of OCT are analyzed in terms of linear system theory. It is shown that OCT has a band pass filter behavior. The system transfer function is compared to the transfer function of a coherent confocal microscope.