Orphan sources may cause serious accidents as history has shown, and in the past few years attention for malevolent use of radioactivity has grown considerably. This paper first describes the context of the problem. We propose to introduce a scheme for the prevention of loss or abuse of sources. This programme includes administrative measures for strong sources, controlling mechanisms and aspects of waste management. Secondly, we address basic requirements as regards monitoring given lost or stolen sources, or if a contamination of unknown origin is identified. The paper pleads for well-trained emergency staff; for the presence of advanced equipments and expertise at national or regional level, and for international collaboration where necessary. The main steps in an adequate response scheme are discussed and illustrated, and the aftermath of the crisis (clean-up actions) is discussed as well. Finally, we stress the importance of measurements in a context of information and openness.