Hypertext-based user interfaces have become attractive for many distributed applications today, but they do not reach the usability level of window-based UIs. Because of insufficient dialog control logic, they cannot manage nested and hierarchical dialog structures that users have come to expect from window-based UIs. We therefore present a framework that implements a dialog control logic capable of handling complex, nested dialog structures, and introduce a notation and an XML-based language for specifying such dialog structures. Key concepts are the encapsulation of multiple dialog steps in context-independent dialog modules that can be nested arbitrarily, and the specification of multiple device-specific interaction patterns for a single device-independent application logic. The framework allows black box reuse, leaving only the implementation of the application logic, the design of the user interface and the specification of the dialog flow to application developers.