Even taking into account our current inability to resolve Open Questions 1–3, the hierarchy of complexity classes depicted in Figure 2 ranging from \(\textbf{P}\) to \(\textbf{EXP}\) represent the most robust benchmarks of computational difficulty now available. Beyond this hierarchy a wide array of additional classes are also studied which are believed to demarcate additional structure either inside \(\textbf{P}\) or between \(\textbf{P}\) and \(\textbf{NP}\). Comprehensive consideration of the diversity of these classes is beyond the scope of the current survey. But to complement our understanding of classes defined relative to \(\mathfrak{T}\) and \(\mathfrak{N}\), it will be useful to consider three additional classes – \(\textbf{NC}\), \(\textbf{BPP}\), and \(\textbf{BQP}\) – which are defined relative to other models of computation.