Examples relate to identifying a signature for a data set. In one example, a computing device may: receive a data set that includes a plurality of data units; iteratively determine a measure of complexity for windows of data units included in the data set, each window including a distinct portion of the plurality of data units; identify, based on the iterative determinations, a most complex window of data units for the data set; and identify the most complex window as a data unit signature for the data set.