Marine ecosystems are essential for the regulation of the biogeochemical cycles and climate on Earth. Nevertheless, the organization, dynamics and evolution of these ecosystems are still poorly understood. Furthermore, we still do not know all the impacts of climate change on these ecosystems and the microorganisms which are living there. For this reason, the Tara Oceans project (http://oceans.taraexpeditions.org) was launched in September 2009 to study planktonic ecosystems in the oceans for 2.5 years aboard the schooner Tara. The project implemented a unique sampling program including the most advanced technologies of optical and genomic methods to describe viruses, bacteria, archaea, protists and planktonic metazoans in their physicochemical environment. More than 30,000 samples of 153 sampling stations around the world have been collected by an international team of scientists. This international consortium of more than 100 scientists includes experts in marine biology, ecology, oceanography, cellular and molecular biology, genomics and bioinformatics from 50 laboratories and institutes. The main goals of this expedition are to study i) the structure of planktonic ecosystems, ii) mechanisms of evolution and iii) populations and communities dynamics in relation to their physico-chemical environments.The results of this expedition will become a valuable resource for future generations.