Book excerpt: WhatsApp is a popular mobile application that was founded in 2009 by Jan Koum and Brian Acton. The app allows users to send text messages and voice messages, make voice and video calls, share images, documents, user locations, and other media. In addition to personal messaging, WhatsApp also offers a range of features for businesses, including WhatsApp Business, which allows small businesses to communicate with their customers and share updates over the app. WhatsApp has grown to become one of the most widely used messaging apps in the world, with over 2 billion active users across 180 countries. WhatsApp makes use of end-to-end encryption to ensure that only the sender and recipient of a message can access its contents. The company has been praised for its commitment to user privacy, although it has also faced scrutiny over its handling of false information and its role in facilitating political unrest. Nevertheless, the app remains a vital tool for communication and connection, particularly in countries where other messaging apps are restricted or banned. Its continued popularity is a testament to the users' trust in its security and reliability.