The United States contains regions of greatly varying seismicity ranging from a relatively narrow strip of very high seismicity along coastal California in the West to broad areas of low or moderate seismicity typical of the Central and Eastern United States. The United States currently has three major regional model building codes. While all three codes have traditionally used the concept of seismic zones to identify and distinguish between regions of different seismicity, they have not had a consistent basis for their seismic criteria. Beginning in the year 2000, the three model building codes will merge and become the new International Building Code (IBC) applicable to the whole United States. New seismic design criteria have been developed for the 2000 IBC that now define ground shaking for building design by spectral acceleration contours. This paper describes the background and basis for the new seismic design criteria of the 2000 IBC, and how these criteria address the large variation in seismic hazard across the United States.