This document introduces Flutter, an open-source UI software development kit created by Google for building mobile, desktop and web applications from a single codebase. It discusses how Flutter allows developers to create high-quality, native mobile apps for both iOS and Android using its platform integrations, simple layout system and hot reload capabilities for fast development. The document also highlights Flutter's large and growing ecosystem, portability across platforms, and use by companies worldwide to build native iOS and Android apps with a single codebase.