Paleolimnology is the study of the early history of lakes based on sediment composition. Lake sediments build up in undisturbed areas of lake bottoms (e.g. in deep central basins) over long periods of time. These sediments will reflect the prevailing conditions in the catchment, climate and in-lake processes at the time of deposition. Specific 'markers' can be used to determine when a particular layer of sediment was deposited. For example, in volcanic areas lakes sediments may contain volcanic ash layers from historical eruptions that can be used to date the sediment (Figure 1). Where it is possible to take long sediment cores in relatively undisturbed areas the historical record of the lake may stretch back far enough to give insights into historical climatic conditions and pre-human settlement state of the lake. This pre-human condition is sometimes referred to as a "reference state". Defining the reference state is important as it provides an indication of the extent to which the current state differs from it. This difference provides a useful reference point for setting targets to improve the state of a lake (e.g., as part of the limiting-setting process envisaged under the National Objectives Framework for Freshwater Management 2011).