The optical undersea system technique has already proven its performance through major realizations of regenerated and repeaterless links. A new development programme has been undertaken so has to obtain within the next few years technical and economic optima in various fields, such as: fibre transmission and mechanical properties, fibre protection in the cable structure, capacity and flexibility of regenerated systems, length and capacity of regeneratorless systems. Today, still newer technics, such as optical amplification or heterodyne detection, are merging very quickly. They would still permit to increase the performance and reduce the cost of submarine systems. They may be introduced in an early stage in regeneratorless links to increase the range of application of these systems. However, they need to be thoroughly evaluated as regards their impact on reliability before being used in submerged equipment.