The MHD instabilities known as Neoclassical Tearing Modes (NTMs), particularly at low rational orders (helicities m/n = 2/1 and 3/2), have been shown to cause substantial loss of confinement in tokamaks, to an extent that may severely hamper the performance of a future reactor. NTMs display a broad poloidal beta (bpol) metastability range, in which they can only be excited by a finite seed island but, once excited, remain unstable. Efforts to prevent NTMs have therefore focused primarily on the containment of seed islands, particularly the most deleterious ones associated with the crashes of long-period sawteeth, such as can be caused by a core fast-ion population. The process by which sawtooth crashes trigger NTMs is only partly understood - while a strong statistical correlation has been established between the duration (rather than the amplitude) of sawteeth and the triggering of NTMs, unknown additional parameters render individual triggering events operationally unpredictable.