MPI / OpenMP mixed mode codes could potentially offer the most effective parallelisation strategy for an SMP cluster, as well as allowing the different characteristics of both paradigms to be exploited to give the best performance on a single SMP. This paper discusses the implementation, development and performance of mixed mode MPI / OpenMP applications. The results demonstrate that this style of programming will not always be the most effective mechanism on SMP systems and cannot be regarded as the ideal programming model for all codes. In some situations, however, significant benefit may be obtained from a mixed mode implementation. For example, benefit may be obtained if the parallel (MPI) code suffers from: poor scaling with MPI processes due to load imbalance or too fine a grain problem size, memory limitations due to the use of a replicated data strategy, or a restriction on the number of MPI processes combinations. In addition, if the system has a poorly optimised or limited scaling MPI implementation then a mixed mode code may increase the code performance.