Paleoclimatology has grown out of multiple physical and biological fields over the past century, but really came into its own after the early 1970's. An early unifying goal was the need to map out how the climate system varied across a wide range of time scales. The theoretical spectrum of climatic variability, proposed by Mitchell [1976], provides a useful theoretical construct for tracking how our understanding of natural climate system variability has improved over the last two decades. Climate varies over all time scales, from seasonal to those associated with the tectonic evolution of the Earth [ Mitchell, 1976; Kutzbach, 1976; Shackleton and Imbrie, 1990]. An understanding of the variability across each of these frequency bands is needed to anticipate the climate system dynamics of the future. Although great progress has been made, particularly in the last decade, it is surprising how many gaps remain in our understanding of the spectrum of climatic variability.