In this section the basic fundaments are explained on which most FS treatment technologies are based. Unit 2.1 aims to provide an overview of the current state of knowledge on the quantifi cation and characterisation of FS, to identify gaps in the existing body of knowledge, and to put these into perspective with regards to FS treatment objectives. Unit 2.2 is divided into three sections, presenting the physical, biological, and chemical mechanisms used for the treatment of FS. Physical mechanisms include dewatering, drying and volume reduction. These are the most widely employed mechanisms in current FS treatment methodologies, and are generally applicable. In the subsequent units (in the Technology section) it is then explained how these mechanisms are put to work in treatment technologies.