The aim of the Global Earth Observation System-of-Systems (GEOSS) is to improve the information available to decision makers, at all levels, relating to human health and safety, protection of the global environment, the reduction of losses from natural disasters, and achieving sustainable development. Specifically, GEOSS proposes that better international cooperation in the collection, interpretation, and sharing of Earth observation information is an important and cost-effective mechanism for achieving this aim. While there is a widespread intuition that this proposition is correct, at some point the following question needs to be answered: how much additional investment in Earth observation (and specifically, in its international integration) is enough? This leads directly to some challenging subsidiary questions, such as how can the benefits of Earth observation be assessed? What are the incremental costs of GEOSS? Are there societal benefit areas where the return on investment is higher than in others? The Geo-Bene Project has developed a "benefit chain"concept as a framework for addressing these questions. The basic idea is that an incremental improvement in the observing system (including its data collection, interpretation and information-sharing aspects) will result in an improvement in the quality of decisions based on that information. In turn, this will lead to better societal outcomes, which have a value. This incremental value must be judged against the incremental cost of the improved observation system. Since in many cases there will be large uncertainties in the estimation of both the costs and the benefits, and it may not be possible to express them in comparable monetary terms, we show how order-of-magnitude approaches and a qualitative understanding of the shape of the cost and benefit curves can help guide rational investment decisions in Earth Observation Systems. © 2008 IEEE.