The flexibility and dynamism of ubiquitous computing systems have a strong impact on the way their security can be achieved, reaching beyond traditional security paradigms like perimeter security and communication channel protection. Constant change of both the system and its environment demand adaptive security architectures, capable of reacting to events, evaluating threat exposure, and taking evolving protection needs into account. We introduce two examples of projects that contribute to meeting the challenges on adaptive security. The first focuses on an architecture that allows for adaptive security in mobile environments based on security services whose adaptation is guided by context information derived from sensor networks. The second addresses engineering aspects of secure ubiquitous computing systems through making security solutions accessible and deployable on demand and following emerging application-level requirements.