SwiftUI is Apple's declarative framework for building user interfaces across iOS, iPadOS, macOS, watchOS, and tvOS, introduced in 2019. Unlike imperative UIKit, SwiftUI describes what the UI should look like at any given state, and the framework handles the rendering and updates automatically. It uses a reactive data flow model where UI components automatically update when underlying data changes, eliminating manual view controller management. The key mental model: views are cheap, ephemeral structs that SwiftUI recreates frequently—state lives elsewhere (in property wrappers like @State or @Observable models), and SwiftUI diffs the view tree to render only what changed. Master property wrappers and you've mastered 80% of SwiftUI.