Ultra-precision machining, as an effective way to obtain components with high dimensional accuracy and outstanding surface smoothness, is developing rapidly with the progress of science and technology. Polymers have unique properties compared to conventional materials like metals, glasses, or ceramics. Particularly, with great improvements in specific strength, specific modulus and surface modification, polymeric materials are possessing broader applications. Relevant requirements are higher in geometric precision and surface quality. Accordingly, ultra-precision machining of polymeric materials is urgently needed, and great efforts are necessary to be contributed in this field. In this chapter, the knowledge about polymers and related processes for making polymer parts are introduced. Then the necessity of ultra-precision machining for polymers is presented by comparing the advantages of ultra-precision machining over conventional processes under current application requirements. The biggest challenge for successfully ultra-precision machining of polymers is viscoelasticity. This property and its affecting factors are introduced. Based on understanding of the viscoelastic behavior of polymers, two representative examples of ultra-precision machining of polymeric materials are provided: cryogenic machining and fabrication of contact lenses. It can be indicated from the examples that viscoelasticity of polymers need to be controlled during ultra-precision machining. Possible methods can be altering environmental conditions like temperature, designing suitable tools or collets, and optimizing machining parameters. Even though challenges remain to be solved for ultra-precision machining of polymeric materials, it can be predicted that a broad future is coming with more input to meet increasing application requirements from various fields.