Acoel flatworms are a surprisingly diverse group of mostly marine animals belonging to the phylum Xenacoelmomorpha that diverged early from the remaining bilaterians. While the phylogenetic position of Xenacoelmomorpha remains debated, the early divergence and many biological features make them an important group for various areas of research. Symsagittifera roscoffensis, also known as mint-sauce worm, has been investigated for over a century and with the advent of molecular techniques, deep-sequencing and imaging techniques has become amenable to various state-of-the-art approaches and studies. In this chapter, our current knowledge on the ecology, life cycle, anatomy and molecular composition are revised at extenso. We end the thorough description of this acoel model system by suggesting areas that need to be investigated in the near future in order to bring the model to the status of other well-established laboratory referents.