The conceptual design of an ocean observing system for the routine, long-term gathering and processing of ocean data useful for monitoring, describing and predicting ocean climate and its variability is discussed. The ultimate aim of the system is represented by four application areas ; atmospheric prediction ; ocean and coupled ocean-atmosphere climate prediction ; state-of-the-art ocean climate assessment ; and model validation. Models are presented as the unifying glue for the system, providing a means for exploiting observed information in many different ways as well as a means for processing complicated and diverse data sets into a form which has practical applications. Monitoring, description and prediction require different supporting environments in order to exploit this potential. The overall objective of the system is broken down into a series of goals and sub-goals roughly aligned with surface, upper ocean and deep ocean applications and with modelling and information management requirements. A prioritization of these goals is presented and it is shown that ordered implementation of the elements supporting these goals will lead to a sensible, staged implementation of the observing system. The research, development and exploitation of appropriate technology is emphasised. Trade-offs and rationalisation across the elements of the ocean observing system for climate and between other climate components and ocean modules is central to the development and successful implementation. The design is presented as the first stage in a constantly evolving and maturing ocean observing system for climate applications.