In recent years there has been a notable increase in the number of research projects engaged in building supra-long (multi-millennial) tree-ring chronologies. Together with a growing awareness of the potential for anthropogenic climate change, this work is shifting the focus of dendroclimatology. Instead of a more traditional interpretation of tree-ring data in terms of annual-to-decadal timescale climate variability the emphasis is increasingly placed on century timescale changes. We review a number of problems with the interpretation of low-frequency climate change in tree-ring derived data. Perhaps the most significant is the high-pass filtering effect of "standardization" techniques commonly used in chronology construction to remove age-related sample bias in the original tree growth measurement data. These techniques effectively remove low-frequency variability and with it the evidence of long-term climate change. Other forcings may also be 'corrupting' the climate signal in the recent period (that used for calibrating the climate signal). Differences in the origin of the samples or changes in site ecology may also impart an inhomogeneity in the response of tree growth through time, hence violating the fundamental assumption of uniformitarianism that underpins proxy climate research.