Interposers are intermediate substrates that provide high-density electrical connections between multiple dies in 2.5D packaging, enabling heterogeneous integration with much finer pitch and higher bandwidth than traditional package substrates. Silicon interposers use semiconductor fabrication to create fine-pitch interconnects (typically 2-10μm line width, 40-55μm bump pitch) with through-silicon vias connecting top and bottom surfaces. Dies are mounted on the interposer using micro-bumps, and the interposer assembly is then mounted on a package substrate with C4 bumps. Silicon interposers enable very high bandwidth between dies—for example, connecting GPU dies to HBM memory stacks with thousands of connections. Organic interposers use PCB-like materials with finer features than standard substrates, offering lower cost than silicon but coarser pitch. Glass interposers are emerging for improved electrical properties. Interposers enable chiplet architectures, memory stacking, and heterogeneous integration of dies from different processes or vendors. Challenges include cost (silicon interposers are expensive), thermal management, and warpage. TSMC's CoWoS and Intel's EMIB are leading 2.5D interposer technologies.