Numerous deer species have been introduced beyond their native range into ecosystems around the world. Their economic value leads to further accidental and deliberate releases and lack of control is contributing to range expansion in Australia, South America and Europe. Despite localised or regional concern, the scale and generality of detrimental impacts have not been widely recognised. We review the direct and indirect impacts on ecosystems and evidence for interspecific effects on native deer. In New Zealand, where large herbivores were previously absent, severe and novel impacts have been found in susceptible forests. Even where ecosystems contain native deer, invasion by taxonomically exotic deer species carries the risk of cascade effects on spatial plant dynamics and forest composition. In Patagonia, introduced deer have disrupted forest composition, whereas in Europe, ecosystem impacts of introduced species can differ from those of over-abundant native deer. Introduced Chinese muntjac (Muntiacus reevesi) within a coniferous forestry landscape in eastern England differ from native European roe deer (Capreolus capreolus) in their distribution of herbivory among differing habitats, and provide much lower rates of endozoochorous seed dispersal. Frequent concern is expressed that introduced deer species may have detrimental effects on native deer and other ungulates, although potential epidemiological effects have not been investigated. Apparent competition, with introduced prey resulting in increased predation rates on native deer, may be occurring between South American huemul (Hippocamelus bisulcus) and southern pudu (Pudu puda). Habitat and dietary overlap is often substantial among native and introduced ungulates, including deer, and exploitation competition is likely. Evidence includes spatial responses of native to non-native deer and negatively correlated changes in population abundance, but demographic mechanisms have not been demonstrated previously. In a coniferous forestry landscape in eastern England, substantial habitat and dietary overlap occurs between native roe deer and high-density introduced Chinese muntjac. This roe deer population has shown a reduction in body weight and fertility following establishment and increasing abundance of non-native Chinese muntjac, compatible with interspecific competition. European roe deer also appear susceptible to competition from larger grazing deer, including native red deer (Cervus elaphus) and introduced fallow (Dama dama). The widely introduced fallow deer may be a particularly effective competitor in sympatry with intermediate or concentrate feeders. There is need for further investigation of potential interactions of introduced and native deer species, and a wider recognition of the ecological impacts of introduced deer.