  The European DataTAG project has taken a major step towards making the
concept of a worldwide computing Grid a reality. In collaboration with the
companion U.S. project iVDGL, DataTAG has realized an intercontinental testbed
spanning Europe and the U.S. integrating architecturally different Grid
implementations based on the Globus toolkit. The WorldGrid testbed has been
successfully demonstrated at SuperComputing 2002 and IST2002 where real HEP
application jobs were transparently submitted from U.S. and Europe using native
mechanisms and run where resources were available, independently of their
location. In this paper we describe the architecture of the WorldGrid testbed,
the problems encountered and the solutions taken in realizing such a testbed.
With our work we present an important step towards interoperability of Grid
middleware developed and deployed in Europe and the U.S.. Some of the solutions
developed in WorldGrid will be adopted by the LHC Computing Grid first service.
To the best of our knowledge, this is the first large-scale testbed that
combines middleware components and makes them work together.
