A security culture is a set of customs and measures shared by a community whose members may engage in sensitive or illegal activities. Security culture practices minimize the risks of members getting arrested or their actions being foiled. While we are trying to stop bad things from happening, our powerful opponents (usually governments or corporations) are working hard to stop us. This guide is about the security measures activists can take to protect ourselves and make our work more effective. This guide mostly focuses on the actions of law enforcement, such as intelligence officers and police, as it usually has the greatest authority to target activists. Law enforcement uses the legal system to harass protesters. Law enforcement investigates and infiltrates groups. Law enforcement disrupts activities. Opponents and law enforcement engage in targeted excessive force and violence. This guide also describes some ways for activist groups to protect themselves, including assessing risk, weigh the pros and cons, manage emotions, be nice and supportive, stop bad behavior, and more.