Polymer blending represents an economical way to produce new materials with improved properties while keeping the cost of production low compared to the development of new materials. Polymer blends are therefore of great interest to researchers because a proper mixing technique can produce a blend whose properties are improved and meet the specific requirements while keeping the production costs lower. The use of polymers is extremely widespread and there is hardly a single industry branch where polymer mixing does not represent a way to produce materials with improved physical and mechanical properties that can be tailored to specific criteria that a material has to fulfil. Further advancements of technology also allow for polymer blends to be used in an entirely new ways, such as usage of polymer blends for drug delivery in medicine. The main problem is that most polymers do not form a homogenous mixture at the molecular level after blending. This is because polymers have an inherent aversion to mixing. The undergraduate thesis deals with causes of miscibility and immiscibility of polymer blends from a thermodynamic point of view, defining conditions in which a polymer pair is miscible of immiscible. Additionally, mechanisms of phase separation are presented, as well as the definition of a glass transition temperature, along with its experimental methods. The thesis also contains some of the methods for improving miscibility by compatibilization, as well as some methods of polymer mixing processes. Examples of polymer blend usage cases are also presented, as well as their properties.