The goal of the Modular Approach to Cloud Security (MACS) project is to develop methods for building information systems with meaningful multi-layered security guarantees. The modular approach of MACS focuses on systems that are built from smaller and separable functional components, where the security of each component is asserted individually, and where the security of the system as a whole can be derived from the security of its components. The project concentrates on building outsourced, cloud-based information services with client-centric security guarantees.