MyOcean is a three-year European project (2009–2012) developed under the GMES umbrella, which aims at the implementation of a service dedicated to the provision of the core marine information requested by users from different marine sectors. This core ocean information is obtained by combination of observations, modelling and assimilation systems. The area of interest identified in MyOcean covers a wide range of applications of the marine field itself but also of connected fields, such as climate research and forecasting, fisheries and fish farming, merchant shipping, safety and security at sea, environmental protection and regulations, coastal areas monitoring, recreational activities, off-shore activities, and defence. The essential variables required to depict the ocean state for these downstream applications include the following: temperature, salinity, currents, sea level, ice coverage and thickness, nutrients, primary ecosystem variables, etc. MyOcean is committed to providing this core information for the daily state of the ocean (real-time), its short-term evolution, and its history over the past (hindcast and reanalysis). The MyOcean special issue presents a selection of scientific achievements obtained as part of the research and development activities carried out to sustain the MyOcean project. We welcome papers discussing MyOcean modelling and data assimilation developments, MyOcean operational systems and products, validation studies, scientific diagnostics and applications. For a reminder of the themes and objectives of MyOcean, please visit the web page http://www.myocean.eu.org.