This review of published and unpublished information demonstrates that offshore wind farms (OWFs) have major effects on the benthos; that is, the seabed flora and fauna. By adding artificial hard substrata to the marine ecosystem, OWFs create new habitat for colonising benthic species, allowing attachment and attraction of hard-substratum species, in ‘the artificial reef effect’. The general exclusion of fisheries further creates flourishing soft-sediment benthic communities. Although wind farms hardly extend the distribution range of hard-substratum species, they may be stepping stones for non-indigenous and nuisance species. Such an increase in benthic diversity, however, is countered by the loss of, disturbance to and/or alteration of the natural seabed. Despite this, it may be concluded that OWFs create local hotspots of benthic diversity, directly influencing the local marine food web. During construction, the biomass of forage species decreases, affecting predatory and scavenging species negatively and positively, respectively. Mobile predatory species tend to leave the area during construction. Once installed, the flourishing benthic communities greatly increase in benthic foraging species and attract predators. The surrounding natural sediments are affected by the deposition of organic matter from the epibionts on the turbine monopoles and scour protection and by the altered predator community. Given that a new ecological equilibrium in the benthic system will develop over 20–30 years, it is arguable whether a return to the pre-construction state following full decommissioning would be feasible or desirable. In contrast, a ‘renewables-to-reefs’ decommissioning scheme involving only partial removal of the wind farm could ensure protection for ecologically valuable sites. While many data already exist, it is difficult to detect significant effects because these are proportional to the degree of change and the changes may take place at different spatial scales. This should be taken into account in OWF monitoring. Benthic communities are of significant ecological and socio-economic importance at a global level. This includes acting as habitat for numerous species at all life-cycle stages, and as a feeding ground for a range of predators. From a socio-economic perspective, these predators can include species of commercial importance. Benthic communities operate both directly and indirectly as food resources for such species. Therefore, the study of the benthic environment around any activity in the marine environment, including offshore renewable energy, is vital to identify potential effects and their significance. All human activities in the marine environment have the potential, by their very nature, to affect its natural structure and functioning. Because of both the direct effects on the seabed and the intimate links between the water column and benthos (Gray & Elliott 2009), the seabed will always be directly or indirectly affected. The effects of offshore wind farms (OWFs) on seabed communities comprise one of the most important elements when considering the potential impacts of such developments, due to the inevitability of effects arising, especially from monopiles bored into the substratum or gravity supported on the seabed, their surrounding erosion protection layer and the installation of cable routes (Wilson et al. 2010). Even developments in floating wind technology still require anchor points and the connection of associated infrastructure, such as inter-array and export cables (e.g. Butterfield et al. 2005; Statoil ASA 2017; see Chapter 1 in this volume). Therefore, an understanding of the ways in which seabed communities are affected by OWFs is vital, in part, so that appropriate mitigation measures can be identified and deployed. A set of key hypotheses have been generated for this chapter: •Changes in seabed ecology as a result of installing a wind farm in the marine environment are viewed as neither positive or negative in ecological terms. but just different. • The inherent variability of the seabed biota and hydrodynamic conditions may prevent the subtle effects of OWFs being detected, in particular in current wind-farm locations around the North Sea. • Hard structures associated with OWFs are available as colonisation sites and ‘stepping stones’ for non-indigenous species. • A focus on the structure of the benthos rather than its ecological functioning does not satisfactorily assess impact. • The effect of a wind-farm structure on the seabed is mirrored by an effect of the seabed and its biota on the structure. • Given the many human activities and pressures, there are in-combination synergistic and antagonistic effects of all aspects of the same development, and cumulative effects of different developments in the same area, which need to be disentangled. • Location provides opportunities (for habitat creation) as well as threats (to the local biota and habitats), and both need to be considered together. • Climate change will increase the variability of an already highly variable system, making it increasingly difficult to detect the effects of the wind farm and its structures.