Network data envelopment analysis is a relatively new subject, with a short history of no more than 20 years since the term first appeared in 2000. However, due to its ability to measure the performance of complex systems, it has been widely applied to real world problems. As a results of addressing such problems, new ideas have emerged which improve the existing methods, and yield better solutions. In the last two decades dozens of models have been proposed, and new models are still being developed. This raises the following two issues: which model to use when faced with a specific problem, and where to go next. While it is impossible to give exact and complete answers to these questions, this chapter provides some useful directions to think about how to answer them. Before getting into more detail, we first discuss the generality of some major classes of models.