This book provides guidance on the assurance of services when there are safety implications of those services. Tangible examples of such services are air traffic control, a maritime satellite service handling emergency distress calls or an ambulance dispatch service. A set of principles for assuring services is given; guidance is then provided on how to apply the principles through objectives and how to address the challenges that may be encountered. Mitigation methods and techniques to address the specific risks of service situations are covered. Analyses are outlined for identifying undesired behaviours of services. The role of service assurance requirements is explained through the service definition, service architecture and the agreements made at service interfaces, often using Service Level Agreements (SLAs). The approach draws upon concepts from modular assurance and illustrates the use of supplementary assurance (‘wrappers’) to manage the variety of assurance positions presented by services.