The nature of elm bark as a lichen substratum is considered; a method of bark pH measurement is described and examples show the effects of the environment on bark pH. A summary of elm taxonomy includes a field key to the identification of elms. An estimation of the degree of restriction of 97 lichen species found on elms is given together with distribution maps of the most restricted species, and a list of lichens recorded on elm. The spread and control of Dutch Elm Disease is discussed. The future of elms and the consequences for the more highly restricted lichen species are reviewed. Bacidia incompta, Collema fragrans, Gyalecta flotowii, G. ulmi, and particularly Caloplaca luteoalba and Cryptolechia carneolutea, are most at risk from the widespread loss of elms.