Ca-Al-rich inclusions (CAIs) are centimeter sized light-coloured inclusions found in carbonaceous chondrites. CAIs consists of hightemperature minerals and are among the first solids which condensed from the cooling protoplanetary disk. Using Pb isotopic data determined on CAIs an age of 4567.2±0.6 million years can be calculated which can be interpreted as the beginning of the formation of the planetary system. However, due to possible disturbations of the Pb isotopic system within the CAIs this age is possibly only a lower limit of the true age. Also an age of 4571 Ma for CAIs has been given, based on Mn-Cr and Mg-Al isotopic data.