Archaea are single-celled microbes that thrive in the ocean. The cell’s outer layer consists of a lipid membrane. Archaea modify the molecular composition of their lipid membranes in response to changing environmental conditions, notably the temperature of the seawater in which they grow. This led scientists to express the distribution of these lipids as a ratio (TEX₈₆) which is calibrated with temperature through studies of modern ocean sediments. These calibration equations can be used to reconstruct past ocean temperatures. Archaeal lipids have also helped scientists understand other environmental and ecological changes in past marine environments. However, many questions about the application of archaeal lipid-based ratios in marine environments remain unanswered. For example, where are these lipids produced in the ocean? Is temperature the only control on the TEX₈₆ ratio? Can we apply calibration equations based on modern climate to all past climate states? This review examines the utility of archaeal lipids for reconstructing environmental change in the ocean and suggests directions for future research.