ONNX Runtime is Microsoft's open-source inference engine for running ONNX models efficiently on any hardware: CPUs, GPUs (CUDA/TensorRT), mobile (iOS/Android), web (WebAssembly), and edge devices. It provides language bindings (Python, C++, C#, JavaScript, Java, Go, Rust), optimization passes, hardware acceleration, and performance monitoring. Inference (running pretrained models) is production's bottleneck. ONNX Runtime optimizes latency, throughput, and memory usage. A well-tuned inference pipeline can serve 10x more requests on same hardware.