Hardware-software integration is the academic concept and methodological approach concerned with the systematic design, development, and validation of computational systems by focusing on the synergistic interaction and co-optimization of their physical hardware components and controlling software layers. It investigates the principles, methodologies, and challenges of combining hardware and software to achieve desired system functionality, performance, efficiency, reliability, and security through cross-layer design, interface management, and holistic system-level analysis, which is critical for enabling complex embedded, cyber-physical, and high-performance computing systems.