Geant 4 is an object-oriented toolkit for simulating the passage of particles through matter. At the very heart of Geant 4 there is a wide set of complementary, and sometimes alternative, physics models which describe the basic interaction of particles with matter. After a review of the main characteristics of the toolkit and a status report of the current activities, the results of a series of detailed tests for the quantitative validation of the electromagnetic models of Geant 4 are presented. Such precision tests demonstrate the reliability of the physics models provided in Geant 4 and are hence particularly relevant for critical applications of simulation models.