The MarClim project was partnership funded by country agencies, for the Marine Biological Association (MBA) to resurvey stations around the UK, where historical data existed. The MarClim project was established in 2001 to investigate changes that had occurred in rocky intertidal systems within the last 50 years around the UK. MarClim established a low-cost network of sites covering England, Wales and Scotland which provided subsequent annual updates to track how climate influences the marine biodiversity of the British Isles. This dataset consists of the Welsh coverage of MarClim surveys only. Since 2005, Countryside Council for Wales (CCW) has continued working with the MBA, through a Memorandum of Agreement (MoA), to continue the work. The annual survey in Wales forms part of a longer, thirteen-year continuous UK survey. Geographical coverage includes sites throughout north, mid and southwest Wales for which historical data dating back to the 1950s exist, and additional sites where range extensions have been predicted to occur. Thirty seven sites were surveyed in north Wales and seven sites in south Wales. MarClim has been highlighted in a global scientific assessment of the need for scientific research to be appropriately designed to provide fit-for-purpose information at relevant spatial and temporal scales useful to managers of protected areas, statutory bodies and policymakers. The main aims at the outset of the MarClim project in 2001 remain as follows: - To use existing historical information and collect new data on intertidal indicator species from the last 50-100 years to develop and test hypotheses on the impact of climatic change on marine biodiversity in Britain and Ireland. - To forecast future marine community changes on the basis of the Met Office’s Hadley Centre climate change models and the United Kingdom Climate Impacts Partnership’s climate change scenarios. The broad range of species known or likely to be temperature sensitive was covered. - To establish low-cost, fit-for-purpose, methodologies and networks to provide subsequent regular updates and track how climate influences the marine biodiversity of Britain and Ireland. - To provide general contextual time series data to support reporting on the success or otherwise of the Marine Strategy Framework Directive, marine aspects of Biodiversity Action Plans, European initiatives including the Habitats, Birds and Water Framework Directives, and management and monitoring of marine activities and resources, including fisheries and Special Areas of Conservation. - To evaluate whether the climate indicator species used in this work have a wider contribution to make as part of the sustainability indicators that are needed to underpin the UK sustainable development strategy. - To disseminate the results widely, and accordingly elucidate the known impact climate has had on marine biodiversity over the last 100 years, and may have in the future. - To provide a basis for the development of a proposal for European Commission funding to establish a pan-European network with related aims. - To assess and report on the likely consequences of the predicted changes in response to climate for society, for commercial and non-commercial users of the marine environment and the policies and frameworks that conserve, manage and protect marine biodiversity. To assess whether any more serious impacts can be ameliorated or mitigated.