This chapter deals with the principles underlying the measurement of safety. It assesses the safety health' of complex technological systems that by virtue of their many-layered defences have relatively few bad accidents. The chapter introduces safety space, which is a natural extension of the resistance-vulnerability continuum. Safety space is a notional space within which we can represent the current resistance or vulnerability of an individual or an organization. An organization's position within the safety space is determined by the quality of the processes used to combat its operational hazards. The organization must possess the technical competence necessary to achieve its safety goals. The chapter looks at the things that could be measured and the likely returns on investment for each of the possible areas. It outlines a workable alternative the regular assessment of the organizational processes that are common to both quality and safety. The chapter discusses some of the principles by which cost-effective safety measurement can be achieved.