On the basis of the available age data from the Baltic shield it is pointed out that the classification of the Precambrian rocks in Sweden must be founded on a time scale of world wide acceptance. This time scale evidently can be based on periods of igneous activity contemporaneous on all continents. Such events seems to be the Grenville-Sveconorwegian period, the Penokean-Svecofennian period and the Algoman-Saamian period, all recorded in the northern hemisphere. The close of each of these periods may be chosen to represent the beginning of a new era. The resulting four Precambrian eras, tentatively named according to international agreement, represent the largest divisions of a time scale within which orogenies and local formations maybe placed by geochronological methods. Thus it is possible to discern in Sweden on one hand Svionian, Jotnian, and Dal formations and on the other Svecofennian and Sveconorwegian periods of igneous activity, and to coordinate these with a time scale established in the above manner.