A series of international conferences and symposia focused on deep mixing alternated between Japan (1996, 2002, 2009) and Sweden (1999, 2005). 2015 marked the first event in the series to be held in North America (San Francisco, Calif.). Important advances in materials, equipment, system design, and project delivery mechanisms have occurred since 2015. Deep Mixing 2021 will provide a forum for international exchange of this new information. In keeping with the focus of this series, the conference scope consists of deep mixing and mass mixing by mechanical means that may be assisted by fluid energy. DFI is proud to again host this event but this time in Gdansk, Poland.