ARKit is Apple's comprehensive augmented reality framework spanning iOS, iPadOS, and visionOS. It provides world tracking, scene understanding (planes, objects), gesture recognition, and spatial anchors. ARKit 4+ unifies the stack: developers build AR experiences that scale from a phone to Vision Pro. Advanced ARKit mastery includes LiDAR utilization, multi-user sessions, Vision Pro spatial computing patterns, and optimizations for both 2D viewport (phones) and unbounded spatial computing (headsets).