Abstract: Mobile communications networks and devices can be and have been used by authorities to warn and instruct the general public during crises. However, our understanding of how mobile technologies could best be used for emergency announcements (public warnings) is currently limited. To clarify one part of this field of study, we define and describe a simple taxonomy for mobile emergency announcements (MEA) systems. The taxonomy has three categories: preplanned MEA systems, ad-hoc MEA systems and semi ad-hoc MEA systems. Differences in functional, security and other requirements were found between MEA systems belonging to different taxonomy categories, both concerning how each category of MEA systems can meet the common requirements, and concerning which requirements are the most important for each category of MEA systems. The differences between the categories were especially clear for these requirements: the understandability and credibility of the MEAs and the security of a MEA system.