Cage aquaculture has grown rapidly during the past decades and there has been a move towards the development and use of more intensive cage-farming systems to access and expand into untapped open water areas, particularly in marine offshore waters. Fish cages vary in design, size and material used as they have been designed for employment in diverse environments, ranging from relatively protected to highly exposed and dynamic sites, either as floating or fully submerged structures. This technica l manual focuses on the high density polyethylene (HDPE) cages as they are widely used in modern-industrial marine aquaculture in many parts of the world. It provides the reader with highly practical and technical information on the design and components of a typical HDPE cage, on how a cage collar is assembled and the net pen installed. Along with the structure of the cage, comprehensive information on the grid mooring system and installation is provided. Finally, information on farming operati ons that includes maintenance and control of the farming structures, stocking of the farmed fish, feeding, harvesting and packaging are discussed along with other practical aspects and routine management operations.