1. This account presents information on all aspects of the biology of Ulmus glabra Hudson (wych elm) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation. 2. Ulmus glabra is a large forest tree, and often an important canopy tree in ancient and seminatural woodlands. It is primarily native to the north and west of Britainand much of mainland Europe. It is the only elm native to Ireland. It is the most distinct of the British elms in that it rarely suckers and sets abundant viable seed. Although found on limestone screes and cliffs, and hedgerows, it is primarily awoodland tree, especially on moist, basic soils. In many secondary woodlands, it often cooccurs with Acer pseudoplatanus and has ecological needs that are similar to Fraxinus excelsior. 3. Ulmus glabra has clusters of c. 25 hermaphrodite flowers appearing before the leaves on previous year’s growth. Seeds are wind dispersed, falling in April to July, but remain viable for only a few days. Nevertheless, seedling establishment can be abundant. Hybridisation with other northern European elms is common but hybrids are notoriously difficult to identify and therefore probably under recorded. 4. The health and survival of wych elm in Europe has been seriously compromised since the 1970s due to Dutch elm disease caused by the fungus Ophiostoma novo ulmi, transmitted by elm bark beetles (Scolytus spp.). To the south of its Scottish stronghold, many elms are reduced to small trees regrowing from basal sprouts or seeds. These trees tend to be reinfected once trunk diameter exceeds 10 cm. Fortunately for its long term survival, seed production usually begins a number of years before they are reinfected.