Once the feasibility of the method was established, the first question addressed was whether plaques are stable or increase in size over time. The evidence so far indicates that the majority of plaques are remarkably stable in size. Although occasional "outliers" show evidence of increases in size, these are in the minority. In addition, Christie provided a good example of new plaque formation within a field that had been imaged on several occasions. Although this evidence appears to suggest that amyloid plaques form precipitously and then remain static, previously published work from this laboratory, led by Bradley Hyman, indicates that the relatively stable size of plaques results from an equilibrium state between an aggregation and degradation process. The in vivo two-photon technique should provide additional insights into the dynamics of plaque formation that, in turn, could reveal aspects of AD. What is more, they could offer a literal window onto the effects of "plaque-busting" therapies in transgenic mouse models.-Keith Crutcher.