Direct Ink Writing (DIW) is an extrusion-based additive manufacturing technology that builds three-dimensional structures layer by layer by extruding inks with shear-thinning properties at room temperature. First patented by Sandia National Laboratories in 1997, the technique is compatible with a wide range of materials including polymers, ceramics, metals, and hydrogels, and does not require high temperatures, making it suitable for heat-sensitive substances. The key requirement is that the ink possesses appropriate rheological properties, and components typically require post-processing such as curing or sintering after printing. DIW can fabricate complex geometries and multi-material parts, showing great potential in energy storage (lithium-ion batteries, supercapacitors), biomedical applications (tissue engineering scaffolds, hydrogels, organ-on-a-chip), and soft robotics. Its advantages include broad material compatibility, room-temperature operation, and hardware flexibility; limitations include slow print speed, limited interlayer interface quality, and primary suitability for R&D and small-scale production. Major manufacturers include Avay, Sygnis, MakerPi, and the open-source platform Printess. For metal processing, DIW opens a differentiated pathway for low-cost, small-batch, multi-material applications.