High precision relative GPS positioning is based on the very precise carrier phase measurements. A prerequisite for obtaining high precision relative positioning results is that the double-differenced carrier phase ambiguities become sufficiently separable from the baseline coordinates. Different approaches are in use and have been proposed to ensure a sufficient separability between these two groups of parameters. In particular the approaches that explicitly aim at resolving the integer-values of the double-differenced ambiguities have been very successful. Once the integer ambiguities are successfully fixed, the carrier phase measurements will start to act as if they were high-precision pseudorange measurements, thus allowing for a baseline solution with a comparable high precision. The fixing of the ambiguities on integer values is however a non-trivial problem, in particular if one aims at numerical efficiency. This topic has therefore been a rich source of GPS-research over the last decade or so. Starting from rather simple but timeconsuming integer rounding schemes, the methods have evolved into complex and effective algorithms.