There is almost no doubt that BIM will become the key technology of AEC (Architecture, Engineer?ing and Construction) industry for next stage.In comparison with CAD, BIM1 is transforming both tools and contents of facility building and operation.Accordingly it's more difficult and complex to understand and implement BIM than CAD.On the other handBIM is expectedto have broader anddeeper impact on the industry.Here the author is trying to provide an answer on the question of " What is BIM" from three aspects: the position of BIM in AEC, howto measure a BIM project and the potential uses of BIM.