TIR is an optical phenomenon. If light is travelling through a medium with a high refractive index and strikes the interface of an optical medium with a lower refractive index at an angle greater than the critical angle, the incident light will undergo total internal reflection. Under these conditions some light still enters the low refractive index medium as an electromagnetic wave termed the evanescent wave. The intensity of this wave decays exponentially with penetration depth. The average z-expansion is less than 200 nm depending on the wavelength of light, the incident angle and the refractive index of the media. In TIRFM, fluorophores in the sample at a maximum distance of ~100 nm from the coverslip surface are selectively excited. This z resolution of 100 nm is one-fifth of an optical section obtained with a laser scanning confocal microscope. Therefore TIRFM is ideally suited for the observation of processes and structures on or close to the cell surface.