We discuss selected issues in palaeoclimatic research, with a focus on tree ring based temperature reconstructions. Topics include the difficulty to retain long-term temperature variations in tree ring based reconstructions, the effects of this and other limitations on the estimation of the absolute temperature amplitude over the past millennium, and the potentials and limitations of including precipitation sensitive tree ring data in large-scale temperature reconstructions. To address these issues, we begin with a brief introduction into some principles of proxy data and specific characteristics of tree ring time-series.