The Mediterranean Sea is currently experiencing a decline in the abundance of several key species, as a consequence of anthropogenic pressures (e.g., increase in human population, habitat modification and loss, pollution, coastal urbanization, overexploitation, introduction of non-indigenous species and climate change). Herbaria and natural history collections are certainly fundamental for taxonomic studies, but they are also an invaluable, if currently underestimated, resource for

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... ecological and evolutionary responses of species to environmental changes. Macroalgae herbarium collections, which are really consistent (ranging from 200,000 to approximately 500,000 specimens) in some European herbaria (e.g., Muséum National d'Histoire Naturelle in Paris, University of Copenhagen, Natural History Museum in Kensington), can be successfully used as real "witnesses" to biodiversity changes. In this respect, we report some case studies from the Mediterranean Sea which summarize well the potential of macroalgae herbarium specimens to provide useful data on biodiversity changes. Indeed, these data enable the evaluation of the responses of biota, including shifts in species ranges, the detection of the presence of introduced species, and the prediction of changes in species distributions and patterns under future climate scenarios. To increase the use of this invaluable tool of research, their curation, the digitization of collections, and specimen genomics should be even more addressed.