URDF (Unified Robot Description Format) is the standard way to describe a robot's physical structure in software. Originally developed for the Robot Operating System (ROS), a URDF file specifies every link (rigid body segment), joint (connection between links), and their physical properties — mass, inertia, collision geometry, and visual appearance. This description is used by simulators, motion planners, and visualization tools to reason about the robot's body.
For humanoid robots, URDF files can be quite complex, describing dozens of links and joints from head to toe. When an engineer wants to simulate a humanoid in Gazebo, MuJoCo, or NVIDIA Isaac Sim, the URDF provides the structural blueprint. It also feeds into control algorithms that need accurate models of the robot's dynamics to plan movements and maintain balance. Variants like SDF (Simulation Description Format) and MJCF (MuJoCo's format) offer additional capabilities but URDF remains the most widely used starting point.
As the humanoid robotics ecosystem grows, standardized robot descriptions become increasingly important for interoperability. Companies releasing open-source URDFs — as Unitree has done for its humanoids — enable the broader community to develop and test control algorithms. The quality of a robot's URDF directly impacts simulation fidelity and, consequently, the success of sim-to-real transfer pipelines. For deeper coverage, see HumanoidIntel.