Polymer-polymer miscibility in the solid amorphous state is reviewed, focusing on homopolymers, random copolymers, and block copolymers. The Flory-Huggins theory is discussed, which predicts that most polymers are not miscible due to the dominance of enthalpy over entropy. However, specific interactions like hydrogen bonding can lead to miscibility. Random copolymers can be tailored to be miscible with homopolymers by adjusting their composition. Theoretical models, such as the interaction parameter based on Hildebrand solubility parameters, help predict miscibility. Block copolymers exhibit unique microphase separation behavior, which affects their miscibility with other polymers. Experimental methods for determining miscibility include observing glass transition temperatures, refractive indices, and turbidity. Polymer-polymer mixtures can exhibit lower or upper critical solution temperatures, with lower critical temperatures being more commonly observed. The literature indicates that about 75% of polymer pairs are miscible due to specific interactions, while 15% are miscible due to copolymer composition. Many polymer pairs show phase separation when all three components (two polymers and a solvent) are considered. The study highlights the complexity of polymer-polymer miscibility and the importance of considering both theoretical models and experimental data in understanding this phenomenon.Polymer-polymer miscibility in the solid amorphous state is reviewed, focusing on homopolymers, random copolymers, and block copolymers. The Flory-Huggins theory is discussed, which predicts that most polymers are not miscible due to the dominance of enthalpy over entropy. However, specific interactions like hydrogen bonding can lead to miscibility. Random copolymers can be tailored to be miscible with homopolymers by adjusting their composition. Theoretical models, such as the interaction parameter based on Hildebrand solubility parameters, help predict miscibility. Block copolymers exhibit unique microphase separation behavior, which affects their miscibility with other polymers. Experimental methods for determining miscibility include observing glass transition temperatures, refractive indices, and turbidity. Polymer-polymer mixtures can exhibit lower or upper critical solution temperatures, with lower critical temperatures being more commonly observed. The literature indicates that about 75% of polymer pairs are miscible due to specific interactions, while 15% are miscible due to copolymer composition. Many polymer pairs show phase separation when all three components (two polymers and a solvent) are considered. The study highlights the complexity of polymer-polymer miscibility and the importance of considering both theoretical models and experimental data in understanding this phenomenon.