This chapter will briefly discuss the science behind using clear aligners in orthodontics. Although fabricating aligners can be made by hand in the lab or using computer aided design/computer aided manufacturing system, the science behind using specific plastic material that has specific elasticity to move the teeth in the intended direction is still unclear and many manufacturers do not declare it as a major manufacturing or business secret. In a simple assumption, the clear aligner materials must be flexible enough to accommodate the existing tooth/teeth position and be stiff enough to move the teeth in the intended direction. Although seems simple, however it took major companies decades spending many millions of dollars on research and development to reach this particular balance. In the following section, a general review of what is available in the literature about clear aligners’ materials is provided.