In the last decade, additive manufacturing's ability to produce customized parts with complex shapes has led to increased global demand for this technology as a manufacturing solution in various engineering fields, from consumer goods, medicine, electronics, and construction to automotive and aerospace. Three-dimensional-printing-based technologies using polymeric materials offer cost-effectiveness, customized geometries, complex design, high-precision manufacturing, and reduced processing time while using highly sustainable materials. This Special Issue aimed to gather the latest research in the field of 3D-printed polymers designed for application in the fields in which this technology attracts the most attention. Papers were expected to take experimental or simulation approaches to challenges in polymer and polymeric composite 3D-printing techniques encountered in each engineering field. Overcoming 3D printing challenges will greatly contribute to advancing development in topics related to transport, engineering, medicine, and the environment. This Special Issue will focus on both fundamental and applied research aimed at the development, characterization, and application of polymers for 3D-printing technologies implemented in, but not limited to, the following: aerospace, automotive, construction, medicine, prototyping, and consumer goods (including electronics, sports, devices, and spare parts).