In most enterprises, the innovation of business services is not something that can happen in isolation. The design and establishment of business services usually result in, or are part of, larger enterprise transformation efforts. The changes brought forward by such transformations may range from the value propositions, via the business services themselves and business processes involved in delivering these services, via changes to the information systems used to support these processes, to changes of the underlying IT infrastructures. Even more, the transformations will typically touch upon several additional aspects of the enterprise, such as human resourcing, finance, reporting structures, etc.Dealing with such profound changes requires a good steering instrument. For senior management of an enterprise, it is paramount to have a steering instrument that enables them to be/stay in control of the direction, progress and impact of such changes. Such a steering instrument should therefore provide enterprises with the ability to analyse the current state of the enterprise, identify and describe alternative future states, analyse the pro’s and con’s of these alternatives, guard the cohesion and alignment between the different aspects of an enterprise (such as business processes and their IT support), as well as monitor the progress and compliance of an actual change program. It aims to provide management with appropriate indicators and controls to steer transformations.When creating an enterprise architecture, several design decisions have to be taken. These decisions are to a large extend based on assumptions about the situation at hand. Such assumptions may pertain to the goals of stakeholders, strategic directions of the enterprise, architecture principles, requirements, etc. In practice, enterprises are confronted with frequent changes and challenges to these assumptions. Even more, the assumptions, and their relative priority, also depend on the specific stakeholders that are involved in creating the architecture of the future enterprise, as well as the actual transformation. This makes it all the more desirable for these organizations to clearly trace any architecture related design decisions back to their underlying assumptions.The aim of the RationalArchitecture project is to explicitly link architecture-level design decisions to their underlying assumptions, with the aim of making the rationalization of these decisions explicit and traceable enough to formally reason about them in terms of a logic based framework. Doing so will enable an explicit reasoning about the connection between the architecture, the associated design decisions, and their underlying assumptions. The driving hypothesis is that having the ability to reason about this connection, will (1) enable consistency checks of the underlying rationales, (2) enable a more precise capturing of design knowledge, and (3) enable advanced impact/what-if analysis when confronted with changes to the underlying assumptions. The RationalArchitecture project therefore aims to produce a logic-based framework to express and reason about architecture related design decisions and their underlying assumptions. This framework is also the innovative aspect of the RationalArchitecture project.The framework will also cater for uncertainties of the underlying assumptions, as well as the interpretation by, and agreement of, these assumptions by the different stakeholders involved in the creation and implementation of architectures. The framework will be specialised further towards architectural design decisions that impact on two key qualities of enterprises and their IT: Information Security and Trust Management and Enterprise Agility.