Neoclassical tearing modes are one of the most serious concerns for operationon a next-step tokamak device. The modes occur on present tokamaks atnormalized pressure (βN) values comparable to those envisaged forbaseline scenarios in future devices, such as ITER-FEAT. Further, empiricalscalings based on data from many of the present machines point to much lowerthresholds on a larger device. However, physics-based models indicate animportant role for the seed island mechanisms, which may in fact give rise toincreased stability on larger devices - i.e. if the seed island width(required to trigger the NTM) falls below the critical levels required. Fitsbased on these models suggest this is the case, but are too badly constrainedat present to make reliable predictions, and the physics is complex, makingquantitative theoretical calculation difficult. Further experiments arerequired to examine the scaling of the seed, as well as to identify the roleand relative sizes of the stabilizing terms that set the critical size formode growth. In the event that the modes are unavoidable, promising feedbackstabilization techniques are being developed with the use of localized RFcurrent drive to change the stability properties of the plasma. Further workis needed to demonstrate sustained access to higher βN andprovide data to refine models. This paper reviews the underlying physics andkey issues, commenting on the present status of understanding and further workrequired.