  With Geant4 Version 10.0, released in December 2013, one of the most widely
used Monte-Carlo codes has been ported to take full advantage of multi- and
many-core CPUs thanks to the introduction of event-level parallelism via
multithreading. In this paper we review recent developments to allow for a
better integration of parallel Geant4 jobs with external libraries. We have
chosen to develop examples using the popular Intel Threading Building Block
(for short TBB) as an alternative parallelization approach to the native Geant4
POSIX. To simplify the scaling of a Geant4 application across nodes on a
cluster we are improving the support of MPI in Geant4. In particular it is now
possible to run an hybrid MPI/MT application that uses MPI to scale across
nodes and MT to scale across cores. %The recent developments allow users to
easily implement parallel application resources that scale on a very large
number of nodes and cores typical of HPC resources.
