This chapter reviews basis for some fundamental techniques, principles, and practices of dendroecology. Dendroecology refers to applications of dendrochronological techniques to problems in ecology. The important ecological problems for which dendroecological techniques are well suited include widespread outbreaks of herbivorous insects in forests, tree decline observed in forests of central and northern Europe and in some areas of the United States, and potential environmental changes brought about by the rising concentration of atmospheric CO2 and other gases. A variety of structural characteristics of tree rings, such as width, wood density, and vessel size exhibit variability from one ring to the next. The principles and practices of dendroecology—namely, uniformitarianism, limiting factors, crossdating, standardization, variance of the mean and the signal-to-noise ratio, sample replication, tree and site selection, calibration and verification, and modeling are essentially the fundamental framework for understanding the discipline of dendroecology. It is noted that these principles are not laws or rules of nature but are well-tested best inferences based upon known facts at a particular time.