In order to develop the knowledge base necessary to design deep sea pressure vessels, it is essential to understand the full chain from design and manufacturing through nondestructive testing (NDT) and characterization to long-term behavior under hydrostatic pressure. This paper describes results from European and national research programs focusing on the use of composites for underwater applications over the last 20 years. Initial tests on small glass/epoxy cylinders were followed by large demonstration projects on carbon/epoxy cylinders with implosion pressures of up to 600 bar, corresponding to 6000 m depth. Numerical modeling has enabled end closures design to be optimized for test performance. Thin and thick wall cylinders have been tested under quasi-static, and long-term loading. Both thermosetting and thermoplastic matrix composites have been tested to failure, and the influence of defects and impact damage on implosion pressure has been studied. These deep sea exploitation and exploration studies were performed for oceanographic, military, and offshore applications, and extensive data are available. The aim of this paper is to indicate existing results, particularly from European projects, in order to avoid costly repetition.