Ubiquitous computing environments offer a wide range of devices in many different shapes and sizes, creating new possibilities for interaction. In the context of meetings and teamwork situations, it is desirable to take advantage of their different properties for synchronous collaboration. Besides providing an adapted user interface, this requires the software to be designed for synchronous access to shared information using heterogeneous devices with different interaction characteristics. The handling of these requirements poses challenges for software developers. As this field is still emerging and no mature models, tools, and standards are at hand, developers have to create their own solutions from scratch. The goal of this thesis is to provide guidance and support for developers of synchronous groupware applications for ubiquitous computing environments. They have to be enabled to develop applications more efficiently and with the flexibility and extensibility that is required for ubiquitous computing. The development effort can be reduced effectively if support for developers is provided at several levels. Developers need assistance when creating models of the applications to be developed, when choosing an appropriate architecture, when creating the design, and finally when implementing. This implies that an architecture-driven, model-based development approach should be followed. While the implementation of a single synchronous UbiComp application still requires research, the development of appropriate development support is even more challenging. Common properties of ubiquitous computing applications have to be identified. Future developments and extensions have to be predicted. Requirements of different research areas have to be fulfilled. Addressing these aspects, the goal of this dissertation is accomplished by providing extensions to the state of the art at four levels: A conceptual model of synchronous UbiComp applications defines a high-level structure for applications that ensures reusability and extensibility of developed software components. It identifies separation of concerns, degree of coupling and sharing, and level of abstraction as the three main design dimensions of these applications. The conceptual model provides two key contributions to the state of the art. First, it proposes the strict separation of user interface and interaction concerns orthogonal to the level of abstraction that is not found in current HCI models. This is a crucial extension of HCI models that is required in the context of ubiquitous computing. Second, it introduces a new view on the concept of sharing. By applying the CSCW concept of sharing in the context of ubiquitous computing, sharing user interface, interaction, and environment state becomes relevant. Thereby, the concept of sharing as known from CSCW can be extended to function as a guiding principle for UbiComp application design. This novel design approach helps ensuring the extensibility and flexibility that is required in ubiquitous computing. A flexible software architecture identifies essential abstractions that support the development of synchronous applications in “roomware” environments. Roomware refers to the integration of room elements with information technology, such as interactive tables, walls, or chairs. Roomware environments represent one form of ubiquitous computing environment. They are used in this thesis as an application context for the conceptual model. The software architecture refines the conceptual model to meet the needs of roomware environments. An object-oriented application framework that has been designed and implemented provides a reusable design and reusable software components. Furthermore, extensibility is supported by explicit mechanisms that are provided to allow adaptability for variable aspects of applications. Thus, the application framework helps developers with the design and implementation. To show how model, architecture, and framework can be applied, the design of sample roomware applications is explained. To demonstrate the extensibility, several new forms of interaction that are required for roomware environments are implemented. The developed applications and interaction forms are used in i-LAND, the roomware environment at Fraunhofer IPSI. Besides being a contribution on their own, the developed applications and new forms of interaction provide evidence that the conceptual model effectively supports developers in meeting the requirements of roomware environments. They show that the model helps reduce the implementation effort when accompanied by appropriate software development tools such as the application framework. The conceptual model, software architecture, and application framework presented in this thesis relieve software developers from the burden of handling all details of multiple interaction forms, and of many critical issues when dealing with synchronous collaboration. By these means, the developer can concentrate on the task at hand designing software at an appropriately high abstraction level, and thus create applications with a higher quality that are flexibly extensible.