The new technologies, such as optical coherence tomography (OCT), for in vivo measurement of retinal nerve fiber layer (RNFL) thickness provide a new venue for the old debate about the relationship between structural and functional damage in glaucoma. Understanding this relationship has both practical, as well as theoretical consequences. For example, knowing the precise functional relationship between two tests, if combined with an measure of their intersubject variability, allows one to predict their relative sensitivity.1,2 Harwerth et al. (121) investigated the relationship between RNFL thickness (measured with OCT) and behavioral sensitivity (measured with static automated perimetry (SAP)) in monkeys with experimental glaucoma. This is an important and unique data set. Unfortunately, this study does not show the actual data (i.e., a plot of SAP loss vs. OCT thickness). Instead, each measure is converted to an estimated loss in retinal ganglion cell (RGC) axons. Thus, it is important to understand the assumptions underlying these conversions. Without going into details, it appears that there are at least three key assumptions that are open to debate. First, they assume, based upon earlier work,e.g.,3,4 that the relationship between the log of the number of RGCs and log SAP sensitivity is linear. Others have argued that this relationship is linear on linear, not log, coordinates.1,2,5-8 Second, they assume that when all RGC axons are gone, the RNFL thickness is zero. In a similar study with human SAP and OCT data, we have argued that there is a residual OCT RNFL thickness remaining after essentially all RGC axons have died.9 Third, they assume that in the normal eye, the SAP sensitivity also varies with number of axons and RNFL thickness, an assumption that needs testing. While the fit of their model supplies evidence for their assumptions, their data need to be tested with alternative models. In any case, this is an important study that speaks to an important aspect of the structure vs. function debate.