This account presents information on all aspects of the biology of Crataegus monogyna Jacq. (Hawthorn) 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. Crataegus monogyna is native to the British flora, occurring frequently in hedgerows, scrubs, thickets and woodland. It can be found throughout almost all of Europe, on all soils of medium conditions regarding pH value, nutrient and water supply. Crataegus monogyna is a deciduous shrub or rarely a small tree of 2–8 m. Its twigs and branches bear sharp thorns about 1 cm long. Crataegus monogyna is of both ornamental and ecological value. During flowering in May and June, shrubs may appear white through a multitude of flowers, presenting pollen and nectar to a variety of different insects. Starting in August, almost the whole shrub can become dark red with the huge number of small red berries (pomes) produced during fruiting. The fruit are a preferred food for many birds. Although hybridizing freely and frequently with the other native species, Crataegus laevigata (Poir.) DC., the two species are easily recognizable in natural stands in the British Isles. Elsewhere, and with the occurrence of horticultural naturalizations and many intermediate forms of hybrid origin with closely related Crataegus species (especially the similar looking one‐styled species Crataegus rhipidophylla Gand. s.l. and Crataegus × subsphaerica Gand. s.l.) expert knowledge is required to avoid misidentifications and thus inaccurate understanding of frequency and distribution–not only on continental Europe but also increasingly in the British Isles. Identification in the field is further complicated by inbreeding of horticultural stock, which suffers from a myriad of descriptions and given names at different hierarchical levels. Cultivars are commonly planted in hedges and along roadsides or for ornamental purposes. The origin of this stock is not always known, so genetic exchange with the natural populations may lead to introgression and thus genotypes that are more adapted than the local genotypes in a changed environment.