3D integration stacks multiple dies vertically with electrical interconnections, enabling higher functionality density, shorter interconnects, and heterogeneous integration of different technologies. Dies are bonded face-to-face or face-to-back and connected through TSVs, micro-bumps, or hybrid bonding. 3D integration provides significant advantages: reduced interconnect length improves performance and power efficiency, smaller footprint enables compact systems, and different process technologies can be combined (logic + memory, different nodes). Memory stacking (HBM, HMC) uses TSVs to achieve extreme bandwidth through wide parallel interfaces. 3D processors stack compute and memory dies for reduced latency. Image sensors stack pixel arrays with signal processing logic. Bonding approaches include wafer-to-wafer (highest throughput), die-to-wafer (known-good-die selection), and die-to-die (maximum flexibility). Challenges include thermal management (heat removal from buried dies), testing (limited access to internal dies), alignment accuracy, and yield multiplication. 3D integration is increasingly adopted for high-performance computing, AI accelerators, and mobile devices.