Book excerpt: In your action group, is it ever beneficial to lie to other members? When is it wise to lie to authorities? If a member of your group has done something wrong, is it better to be open about it now or keep it hidden in the hope that outsiders will never know? What are the pros and cons of infiltrating opposition groups to collect information about harmful activities? Should we wear masks at rallies? There's lots of research showing that lying is an everyday occurrence in most people's lives, and furthermore that lies can be beneficial in some circumstances. But they can also be very damaging, especially lies by authorities. The Deceptive Activist introduces key ideas about lying and deception and then provides a series of case studies in which activists need to decide what to do. There are no final answers, but it is important to address the questions.