With the worldwide spread of clear aligner treatment (CAT), a plethora of new thermoplastics is currently commercially available on the market, claiming to have superior properties and greater comfort. This review aims to summarise the properties of the materials and their effects on treatment effectiveness and comfort to ease material selection and also incorporate new emerging trends such as shape memory polymers (SMPs) and direct 3D printer aligners. First, a concise historical overview of orthodontics will be presented, along with the basic properties of thermoplastics and their importance in treatment. Following the individual properties, we present an analysis of optical, biocompatibility, and toxicity aspects, passing through others such as thermal, mechanical, and special methods to simulate in vivo measurements. We end with the impact of this technique on the environment and the advances and perspectives of CAT.