This presentation provides a summary of the Service Design chapter of the upcoming SOA Made Simple book. Services are key a key aspect of any SOA. Once you have identified what services your clients require, you can start designing these services by designing their operations, and the input and output of these operations. Service design principles indicate what qualities a service needs to have in order to be a usable building block in the architecture you are trying to achieve. When services are poorly designed or poorly implemented, your solution architecture will probably have little value for the business as well. What we need are sound design principles that help us as a service provider to create (re)useable services, and help us as service consumer to judge if the services that we use (or want to use) are well designed ones. This presentation includes several service design principles and quality-of-service aspects that can be used as checklist when creating, buying, or reviewing services. Such design principles include isolation and idempotency. Most important is that services need to be easy to use, must provide value, and that they can be trusted by (future) consumers.