Animation in Unreal is driven by Skeletal Mesh components paired with Animation Blueprints. An Animation Blueprint is a specialized Blueprint that contains an AnimGraph and an Event Graph, blending state machines, blend spaces, and animation graphs to drive a character's pose. Inside the AnimGraph, a State Machine organizes states such as Idle or Run, connected by transition rules based on speed or other variables. Blend Spaces interpolate between multiple animations based on input values; a 1D Blend Space uses one axis (commonly Speed), while a 2D Blend Space uses two axes for walking, running, and directional movement. A Slot node allows animations such as reloads or attacks to play on top of the base state machine without overriding full-body locomotion. Two Bone IK and Full Body IK nodes adjust intermediate joints so end effectors like hands or feet reach target positions.
Anim Notifies are events placed on the timeline of an Animation Sequence that fire functions at specific frames, used for footstep sounds, particle effects, hit detection, or damage application. Montages are reusable animation sequences that play on a slot, optionally blended with the base state machine, commonly used for attacks and upper-body actions. Animation Montages can be triggered from Blueprint through Montage_Play on the Anim Instance, with Montage_Stop ending them programmatically. Skeletal Mesh Sockets are attach points added in the Skeleton editor, used to attach weapons or props to specific bones like a character's hand. To retarget animations between skeletons, both skeletons need compatible bone hierarchies, a Retarget Manager is set up, bone mapping is defined, and a Humanoid Rig is assigned before retargeting assets from the source to the target.
AI in Unreal relies on Navigation Mesh, Behaviour Trees, and Blackboards. A NavMesh is automatically generated from a level's collision geometry within a Navigation Mesh Bounds Volume and provides walkable surfaces for pathfinding. A Behaviour Tree is a visual scripting system that organizes tasks via selectors, sequences, decorators, and services, with AI Controllers running them to evaluate conditions and execute actions like moving, attacking, or idling. A Blackboard is a data asset paired with a Behaviour Tree that stores named keys such as TargetActor, HasLineOfSight, or HomeLocation, which tasks and decorators read and write to coordinate complex AI behavior. The Significance Manager assigns importance scores to actors based on distance, angle, and custom rules, allowing less important AI or animation work to be throttled to maintain frame rate on dense scenes.
Multiplayer in Unreal relies on a small but specific set of classes. Game Mode runs only on the server and defines the rules of a match, including default Pawn class, Player Controller class, HUD class, Game State, and spectator settings. Game State replicates to all clients and stores match-wide data such as time, score, and team information. Player Controller is possessed by each local player and handles input mapping, possession, HUD display, and camera interfacing. Blueprint variables are replicated by setting their Replication property to Replicated, allowing only the server to write them while automatically pushing updates to all clients. The Game Instance persists across level loads for the entire game session, used for save data, player profiles, and persistent UI state that must survive map travel.