During the last years a new optical technique for measuring intraocular distances, partial coherence interferometry, was developed. It is based on the special coherence properties of multimode semiconductor laser diodes or super luminescent diodes. A special dual beam version of this method eliminates any influence of longitudinal eye motions on the results. It was shown that the axial length of the eye (the distance cornea - retina) can be measured with a precision of 20 micrometers in human eyes in vivo. This technique has further been improved: In case of well defined, sharp boundaries between two layers of different refractive indices (e.g. the internal limiting membrane), it is now possible to determine the absolute position (with the cornea as the reference surface) of individual retinal layers with a precision (standard deviation) of a 5 micrometers in human eyes in vivo. The thickness of fundus layers can be measured with the same precision, provided the reflectivity of the corresponding interfaces is high enough to separate the signals from noise. By performing these measurements to different points of the retina, the contours of retinal layers can be determined and thickness profiles of individual layers can be obtained with a precision down to 5 micrometers .