Comparative analysis of molecular profiles (typically gene expression) can identify highly robust patterns that occur most consistently among multiple experiments. A signature can thus be defined as a common pattern, like a fingerprint, reappearing in multiple profiles. Because they incorporate multiple profiles, often from disparate experiments, signatures have improved statistical power and are hence more reliable. Comparing multiple profiles can uncover which signatures are shared and which are unique, for instance in different tissues, different species or across pathological states. In the context of ageing it is of particular interest, for instance, to identify signatures common across tissues or even between species (public mechanisms) as well as those shared by multiple lifespan-extending interventions.